Having done the others, I thought I'd take a crack at Changelings. I'll readily admit that this is a little precipitous; there's just not enough info out there yet to get a good idea of how many Changelings there are. Consider this article tentative, until I've had a chance to play and run the game some more...
Whitewolf has yet to put out much information about Changelings, so much of what I write here is *very* tentative. One thing about Changeling population seems clear, though. In order for them to have the elaborate social structure that they do, they must be fairly numerous. I make a few assumptions about recent changeling history to help account for this large population. Since we know more about the kith in North America than anywhere else, I'll use it as a starting point.
The Kingdoms of North America
We are told that there are 7 kingdoms in North America, all under the High King David. One of these kingdoms is in Canada, one in Mexico, and the other 5 in the US. We also know that Changelings have a hierarchy of Nobility: King - Duke - Count - Baron. We can use this as a basis for computing Changeling Demographics.
Take a typical kingdom. The king will control a region roughly equal to 10 States. Under the king, there will be 3 to 5 Dukes each holding land roughly equal to 1 or 3 States. Under each duke, there will be 2 or 3 Counts, as well as another 3 to 5 Counts that hold their land directly from the king. Each Count will rarely control as much as an entire state, more often a half to a third of a state. Under each count, there will be 1 to 3 Barons, and a number of Barons that hold land directly from a Duke, or occasionally the king. A Baron will hold territory equal to an important city or region. Finally, there will probably be around a half a dozen "rogue" nobles, often commoners or unseelie, that refuse to acknowledge the power of a kingdom.
Note that each Noble will hold personal territory slightly larger than that of the noble of the next lower rank. Thus, a Duke will hold a good portion of an entire state as his personal domain, with a number of Barons directly under him to administer it. A king will personally hold land equal to 2 to 4 States, administered by several Counts that owe him personal homage, and will hold an entire state still more directly, split amongst 4 to 8 Barons.
Typical Changeling KingdomNoble # Land Personal Holdings Immediate Vassals King 1 10 States 3 States 3 Dukes 4 Counts 6 Barons Duke 4 2 States Most of a State 2 Counts, 3 Barons Count 12 1/2 State Major City 1.5 Barons Barons 27 City/Town the same Knights and Squires only Rogues 6 City/Town the same Knights and Squires only Total 50Knights and Squires are rarely "landed", and therefore do not figure into the above totals. Even if they do hold land, it will rarely be more than a single small freehold. Most Barons will have 1 to 3 Knights under them, while Counts will have 3 to 5, Dukes 5 to 7 and the King a dozen or so. Each noble will have an equal number of Squires, half of whom are "Knights in training", and half of whom are honored commoners, who are unlikely to advance further.
Most of the higher Nobility are likely Sidhe. In fact, we know that there is only one non-Sidhe king. Probably 2/3 or all Dukes are Sidhe as are 1/2 of all lesser nobility, down to squires (who are about 2/3 commoners). Despite appearances, about half of the Sidhe have no rank at all (being scholars or independent).
Finally, each noble will have a total of about 40 commoners under them, maybe 10 within their household, and another 30 or so scatter out within their Personal Domain. The Barons will have slightly less (30 or so), the Dukes and Kings more (60 and 100, respectively). All told, this makes about 2000 Commoners, 300 Knights and Squires, and 50 Higher Nobles, for a total of 2350 Kith in each kingdom.
This makes for about 16500 known kith in North America. Throw in the Dauntain, the Nunnehi and the hidden Shadow Court, and the total likely reaches 20,000. There are, all together, about 400 million people in North America, so that makes Changelings about 1 out of every 20,000 people! Bear in mind that unlike vampires, this population is not evenly distribute amongst the human populous. Some areas will see much higher concentrations of kith, and other areas no kith at all.
Changelings in Other Regions
Whitewolf has given even less information about Changelings elsewhere. I would imagine that the 1/20,000 ratio would hold true in Europe as well, making for about 30,000 to 40,000 kith, split amongst a dozen or so major kingdoms (which would resemble, but not exactly coincide, with human nations). My impression is that the Unseelie move more openly in Europe, and fully a third of the kingdoms would be ruled by an self- proclaimed Unseelie Changeling. There is likely no over-king, and there are probably several dozen minor kingdoms that recognizes no overlord.
In other regions of the world, my feelings are that the native Fae would be very different from those presented in the Changeling rules, and they would be quite different in structure and numbers from the descriptions I have given here. Possible exceptions are the ex-European colonies like South America and Australia. There might be a total of 75,000 to 100,000 Western-style changelings world-wide.
Note: The following is largely speculative, but helps "justify" the large modern Changeling population.
Things were very different prior to the return of the Sidhe. There were somewhere between a half to a third of as many Changelings as there are now. Changelings were as rare as 1/50,000 people (or even less), and many were mad, or knew little of their brethren. Most of the commoner kingdoms were small, their boundaries not extending beyond a single city or state, and with a few hundred to as little as a few dozen kith under their control. Half of all kith lived under no kingdom at all.
After the Sidhe returned, and the Accordance War was finished, the various kingdoms were reorganized and expanded to fit older patterns. In addition, Changelings of all types, even the commoners, began to appear in larger and larger numbers, until the kingdoms grew to their current size. There are various theories as to why this might have happened.
The most widely held belief is that prior to the resurgence, many kith never gathered enough Glamour to reach Chrysalis. They went through their entire lives trapped in a mortal shell, never exhibiting their true Fae nature. Other awoke to their Fae nature, but without the support of Changeling society, quickly went mad. Who knows how many of those locked away in asylums were in fact Changelings in Bedlam? With the greater presence of Glamour and a more organized Changeling society, more of these lost kith are able to discover their true nature.
A less popular but more intriguing theory is that some of the commoner kith appearing today are in fact additional exiles from Arcadia. The Sidhe insist that it was only their noble houses that were expelled from the Summerlands, but their memories are hazy. It is possible that some of the more recent commoner kith have only just arrived from Arcadia. If this is the case, the mists have erased their memories so thoroughly that they are nearly impossible to distinguish from other commoners.
A Typical Barony
Most towns or cities that have a Changeling population will be a Barony. The highest noble will be the Baron, who will usually owe homage to a Count, or more rarely a Duke or even the King. The Baron's household will consist of 1 to 3 Knights, and probably a Squire that is training to become a Knight. He may have one or two other Squires that are in fact Commoners raised up to help him administer his land (like the "Reeves" of medieval times). His household will have another half-dozen members that are "commoners", some of whom will be Sidhe that hold no rank, either because they are too young or because they have no interest in politics. Many barons will have a dozen or so enchanted humans, some of them Kinain, to help them run the freehold and do mundane chores.
Baron will probably have the most powerful freehold in the region (often Level 2 or 3). There are likely to be two or three other freeholds (of Level 1 or 2) as well, possibly held by a Knight or Squire, but more likely held by commoners. Each freehold will house 3 to 5 Changelings; at least, that is many Changelings will visit it on a regular basis. There will be another dozen or more changelings that live on their own, not attached to any particular freehold. The entire Changeling populous will likely number 30 to 40. There may be an equal number of enchanted humans and dreamers that regularly interact with the kith.
The Changelings will know and interact with most of the neighboring Baronies. There are probably another two or three Baronies within easy driving distances (a couple hours). One of those nearby "Baronies" is likely to be held by a Count or Duke, probably the Baron's lord. The local Changelings will have a fairly good idea of the general outline of the kingdom they live in, but will be vague on details. The other kingdoms will be known in name, only. The High King David is known to all. The Baron and his knights will probably be better informed, but not to a great degree.
This description applies to a typical rural Barony, or a Barony in a small town. Large cities are typically held by at least a Count, and often a Duke. In fact, in very large cities (like San Francisco) there may be several territories. Also remember that some areas and cities have no changeling presence at all.
Tweaking the Numbers
I have a feeling people will have one of two reactions to the above numbers. I have to admit that when I finally got the 1/20,000 figure for Changelings, I felt that they were too common. Further thought made it clear (to me at least) that it would be pretty difficult to have any sort of pseudo-medieval society without those sorts of numbers, though. I finally settled on using an existing group of pseudo-medievalists, the SCA (Society of Creative Anachronisms) as a rough model.
I also had to assume that for the most part, Changelings have little real impact on human society. That is, most kith devote themselves to Fae things, and leave the complex scheming and society manipulation to the likes of vampires, werewolves and mages. This isn't too unlikely either, given that most Cantrips will just fall apart or have very little effect when used on a Banal human, and Chimera will have no effect at all.
Aaah! There are Changelings everywhere!: If you think that changelings are too numerous, you can tweak the numbers downward most easily by (a) decreasing the number of nobility and/or (b) decreasing the number of commoners in each fief. If you do so, however, you may want to assume that Changeling population is *very* concentrated, so that you have reasonably sized Baronies scattered over great distances.
What! There aren't enough Changelings to fill a football stadium, much less a kingdom!: Increasing the number of changelings should be even easier. I wouldn't recommend increasing the number of higher nobility by too much, but you can easily add more commoners, knights and squires. I wouldn't do much more than double or triple the Changeling population though. The numbers are pushing suspension of disbelief as it is.
There aren't enough nobles!: This is one reaction that I also expect, given that playing nobility can be much more attractive than playing a commoner. There are two ways to handle this. First off, you can easily add more knights and squires to the population. Secondly, you can add "honorary" nobility, that is, nobility with titles but no holdings. Individuals might be given various nobles ranks to honor great deeds.
In addition, children of nobility will automatically have ranks a little lower than their parents. The heir apparent will hold a rank one lower than their parent, while non-inheriting children will hold ranks two lower than their parents. Thus, a prince (son of a king) would be equal in rank to a Duke if he is the heir, or a count if he were not. The Daughter of a Count would equal a baron if she is to inherit, or a knight if she is not, and so on. This system applies to adopted heirs as well as natural children.
This section is also largely speculative, but should be included in any discussion of Changeling demographics. Changelings believe that their Fae souls are eternal, and move from one mortal shell to the next. There are some patterns to the appearance of the Fae souls. When a Changeling breeds with a human, their children acquire some Fae blood, becoming Kinain. The Changelings believe that such children are much more attractive to the Fae spirit, and are thus much more likely to become Changelings.
Changelings are not like Garou; their children are fairly likely to "breed true". Slightly less than half of Changeling/mortal children become Changelings, and most of Changeling/Changeling matings breed true. Those that aren't Changelings are usually Kinain. Kinain that breed with normal humans sometimes pass on the blood and sometimes not. They can also pass on the blood in a dormant state, so that it will skipped generations. Long forgotten lines of descent will on occasion produce Fae children.
There are two basic family patterns amongst Changelings. A third to a half of all Changelings live in interbreeding families of Fae. Most of their children become Changelings, and those that don't are Kinain. They are raised with knowledge of their heritage, and the expectation that they will eventually reach Chrysalis, and become full Changelings. In fact, a few are born in Chrysalis, and never spend any time as a mortal.
The remaining Changelings come from lost lines of descent, lingering traces of Fae blood in otherwise mortal families. Every so often such mortals will be born Kinain, and slightly less often they will fully express their Fae nature and become Changelings. Modern Changeling society watches for such lost children. As the Glamour of Chrysalis is often readily detectable from several miles away, Changelings can seek the forgotten children out and bring them into the fold. Before the Resurgence, many such children were simply lost, and never fully understood their Fae nature.
Another factor in Changeling breeding is that there are no "half-breed" Changelings. If a Troll and a Satyr mate, the child will be either a Troll or a Satyr (or Kinain). The other Fae blood can linger, though, and later generations can express that latent blood of other kiths. If a Troll and a Satyr breed, producing a Satyr, then the child (or grand- child) of that Satyr could well be a Troll. For the most part, kith stick to their own when breeding, but there has been a fair amount of mingled blood over the centuries. Thus, every so often, a loving pair of Redcaps will have a Boggin child!
Sidhe are an exception to much of the above. In 1969, the Sidhe just "appeared" in mortal bodies. Some of them took over already existing mortals, while some just appeared with bodies from who-knows-where. Since that time, they have had breeding patterns similar to other Changelings, but they do not have the mingled blood and lost lines of descent that other Changelings do, at least not yet.
Every so often, Sidhe still appear from out of nowhere. Such Sidhe are much like the forgotten children of other Changelings, though somewhat less common. These random appearances are widely believed to be further exiles from Arcadia. Some believe that there are still Sidhe being kicked out of Arcadia. Others believe that because of the differences in the flow of time in the mortal world and Arcadia, that the exiled Sidhe are arriving scattered over several decades, even though they all left Arcadia at roughly the same instant.
There has been only one or two generations of Sidhe children since the resurgence, but in every way they match the patterns of other Changeling breeding. Given the Sidhe's belief that their souls do not reincarnate, this presents some metaphysical problems. Some Sidhe believe that the new Sidhe are further exiles from Arcadia. Others believe that the new Sidhe children are in fact brand new souls, either created from whole cloth, or somehow co-opted from the human pool of souls. There are those that say this may be one of the reasons why the Five Houses returned to earth, to replenish the dwindling pool of Sidhe spirits.