Are there any canon hints as to how many White Council Wizards are in a typical large city (or as a fraction of the total population)? [
Well, Council-level talents are about one in a million. But it would depend on whether they tend to cluster in cities or not.
(The Council has about 5000 members IIRC. If they're evenly distributed, that's about 220 Council members in the US. If Council level talents are one in a million, presumably the extra thousand or two Council-level talents are warlocks, necromancers, people 'under the radar' like Elaine, etc.... so maybe 80 or so of those in the US. So yeah, Council-level talents are rare.)
In the books, sometimes it seems that Harry Dresden is the only Wizard in Chicago. At other times the landscape seem to get a bit crowded, where you couldn't heave a brick without hitting some kind of sorcerer.
I kind of get the impression Harry is the only full wizard who actually
lives in Chicago, but that *may* just be because most wizards seem to be pretty secretive.
Sells and Kravos weren't full wizard talents. Most of the Kemmlerites seem to have just come in because the NN boundary was already disrupted there -- though we've seen Cowl three times, so he
might be more local.
I think numbers go up quickly as power levels decrease though. Sorcerers don't seem all
that rare, and Paranet types seem very common -- though the number of *active* lesser talents seems more to be a matter of training; the Alphas don't seem to have been conscious of any magical ability before they met Tera West. (And then you have even-lesser talents like the guys from Day Off, who have practically no ability but claim more...)
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But really, the demographics aren't crucial, as luminos says; if you're playing a higher power game, you will probably want more council level NPCs.