As per question in the title.
I recognise that there are numerous criteria that would affect this, however, I am looking for a baseline estimate that could be modified as per specific vampire (his humanity, feeding habits, discipline usage etc.) I would assume that the said vampire is drinking about 2-3 points of blood per night and never kills his victim. However, I struggle to estimate anything else than a herd size (which is composed of exclusively fit-for-purpose healthy people). Assuming that it takes about four months to recover from a loss of a point of blood without any detrimental health effects (implied by the facts that a point is about a pint, that is about a single blood donation and the medical recommendation is to donate blood at most three times a year), I propose the following calculation:
3 * 1 * 4 * 31 = 372 people required to every night feed on 3 different people, drawing 1 point of blood for four months of 31 days, so that no person is bitten twice in this period.
But that doesn't explain how many "real" people would be necessary to at all times have no less than 372 healthy specimens for an indefinite amount of time (self-sustaining population).
Could you explain how to get that number and what would be the major contributing factors?
If this question is not fit to appear here, I could possibly ask in some medical SE instead. Please comment if that's the case.
Some people ask me for purpose, let me explain. That's not supposed to be a tool to estimate the vampire population in a given city. Instead I'm thinking about a single vampire living in a remote area without a practical possibility of leaving (think Tzimisce in a settlement surrounded by werewolves in the middle of Siberian taiga). I am trying to establish how many people should such a vampire have around him to survive by purely local means. I want to take into consideration aspect like not everyone having being suitable to be fed upon, naturally occurring illnesses, necessity of children to replace the elderly etc. to create a situation where a trapped Cainite struggles to keep himself alive. There are some imminent dangers to him, hence his usage of disciplines (additional 2 points per night) but he is aware that overfeeding would steer the population to a decline. I would like that status quo to last for more than five decades. No, I don't want to factor in the social problems of people gradually noticing his presence, I'd rather think of superstitious community who treats it as a curse over a town that has no explanation and him being extra careful to influence mortals into having a lot of children and not leaving, as a local prince could in the medieval times - only in contemporary setting.
Mortals are not supposed to be willing participants, if it were like that, I'd just go with Herd Background and assume no overhead of spouses, children and elders is needed.