Vampire: Dark Ages claim that a city with 5k souls can sustain a descent cast for vampire, however, a number of them would be forced to hunt in nearby villages and settlements. The most convoluted "by night" scenario I ever read had 40 npcs, being 2 of them os a lesser bloodline - which I wouldn't consider using for a game in smaller cities - 3 of them pander/caitiff - which, again, use 1night only shovel heads, not permanent kindreds - 4 of them elders that were added later when the writters realized they needed a plot device for the scenario to make sense - that wouldn't be needed in a less convoluted scenario - and one of them a antediluvian that, franquily, could have enough blood to just not need to feed for months if he so desired. that leaves a minimum amount of 30 well defined NPCs + players for a story to work... assuming every vampire is accounted for. Assuming a standart game of 5 players, that would be 5000/35 around 1 vampire for every 160 mortal IF some of them hunt outside OR count on turists to feed on.
Now, would that make sense in real life? incredibly enough, it would. let's take a look at military organization: a Squad (8 men) was originally divised as the man that share a tent/dormitory/room; a Platoon (3~6 squads men) is the amount of units a man can keep close track and command at any given time; a Company (2~4 plattons) are the men that keep you company, that is, the amount of people you can keep update on how is there life going, what they have being doing, what they like, what they don't, what they think about something etc, and it is somewhere around 120 and 150 people.
As we can see, a working vampire cast is just above the number any individual can keep track of others, that is, just enough so that each isolated vampire wouldn't, necessarily, have someone noticing something weird about them, and that is assuming they go around hunting in nearby villages OR preying on turists (and that is my saving grace for having a chronicle happen in Jackson Hole, nearby villages and turists). Now, there are some factors that shape this numbers, for example, a high violence rate can help cover mass feeding, while a relatively peacefull society would notice any sign of people appearing without a single drop of blood in their veins. similarly, the smaller the city the more likely it will notice weird events, such as an open use of Disciplines, or open warfare between sects. Now, if you are wondering about werewolves, remember that they also have a "masquerade" (the veil, they call it), so, while those unfortunate enough to have to hunt outside would do well to carry some silver on them, it is unlikely that a pack of werewolves try to take down a vampire in downtown, for example. Well, at least openly.