I am trying to figure out how large the population of supernatural beings can be in a stereotypical urban fantasy setting before their numbers stretch the suspension of disbelief. I.e., your standard setting where supernatural beings live among humans under some sort of masquerade system.
Figuring out how big your supernatural society can be is obviously important for plot purposes. Make the supernatural population too large (say, 1 in 25), and it becomes ridiculous that the supernatural could be kept secret because everyone would know at least someone who is supernatural. Make it too small (say, 100 people worldwide) and you dramatically restrict the possibility for narrative conflict because you have at most one vampire/werewolf/whatever per large city.
The World of Darkness gives an oft-cited statistic that there is about 1 vampire for ever 100,000 humans. The best I could figure for a reasonable estimate would be a population ratio comparable to the prevalence rate for a rare disease: something rare enough that almost no one knows someone personally that fits the criterion, even though the disease itself is real.
I realize that in reality there is probably no way the supernatural could keep a reliable masquerade from humanity for any length of time. There have been studies that suggest even the best-kept conspiracy (which would be analogous to, in this case: insta-fail if any evidence of supernaturals got out, complete unity by the supernaturals in wanting to stay hidden, and no supernatural-hunting organization of muggles that want to expose them, all of which is highly unlikely) could only last a few decades at best. Even in these situations the best-kept secrets would be at best like the mafia: everyone knows the mafia exists, but few people know who the individual members of the mafia are or their rank within the organization. I am more looking for an estimate of the largest population possible that would not also break the reader's suspension of disbelief because there is no way a subpopulation of that size could be completely unknown to society at large.
- The supernatural beings have both a distinctly inhuman form (or cover-blowing superpowers) and can also pass for human to most people. Think your standard urban fantasy vampires, werewolves, wizards, faeries, djinn, etc. Or even better, the Wesen from Grimm. So it's not a case of "they can pass for human even when using supernatural abilities".
- The supernatural beings in question do not feed on humans. Therefore, their population is not constrained by the availability of prey, like it would for something like vampires.
- Similarly, because they can reproduce with humans or can reproduce non-sexually (i.e., vampire or werewolf bites) inbreeding or difficulty in finding mates to continue the species is not a concern
- There are no "secret hidden cities" or "town where everyone is supernatural but they pretend to be normal to outsiders" that the supernaturals can set up in the boonies, they are intermixed with the human population.
- For the purposes of this question, it only matters that the general public be unaware of the existence of the supernatural. Muggle Best Friends or family members are allowed to know, the existence of the supernatural just can't be common knowledge. The government is also allowed to be aware of supernaturals and complicit in aiding the masquerade. Obviously if the government did know about supernaturals and was abetting in hiding their existence, it is only a matter of time before government corruption/incompetence/international realpolitik or WikiLeaks spills the beans to everyone, but still. If you want to go ahead and give separate estimates for population sizes if the government is abetting the masquerade (and hence can make evidence disappear) be my guest, but you don't have to.
- No mind whammies, glamours, or flashy thingies to make people forget about the supernatural. If muggles see evidence of the supernatural, the only way the supernaturals have to make the evidence disappear through begging, bribery, vandalism, or threats/murder.
- There is no magical "weirdness censor" that will make humans just plain refuse to believe in the supernatural to the point of active denial. The ability of humans to disbelieve the supernatural is about the same as it is in real life. An isolated paranormal event will be written off. A single person spouting nonsense will be seen as crazy. Repeated sightings of a creature in the woods will draw in cryptid enthusiasts. But strong enough evidence that can't be dismissed by a dedicated skeptic won't be ignored and people will notice when things are going on, even if they can't peg it as supernatural. What constitutes failure in this scenario is the supernatural being treated as "real" and common knowledge by your average person, rather than just an urban legend.
- The majority of the supernatural community wants to maintain the masquerade. Individual exceptions that believe the masquerade is oppressive exist, but they're a small number and are mostly taken out by internal self-policing by the supernaturals (read: staked in a dark alleyway). The supernatural community is mostly unified on this issue.