And you’ve got people. Billions of people walking around like Happy Meals with legs. —Spike
Consider the vampire as a creature bound by the Law of Conservation of Energy.
According to the MadSci Network (how can I not trust these guys?), there are about 700 [kilo]calories in a liter of human blood. A vampire who sucks dry a human adult with five liters of blood is going to get 3,500 calories. One who believes in sustainable humaniculture, drawing at a rate no more than what the Red Cross recommends for whole-blood donors, will get half a liter, i.e., 350 calories, every other month.
So if the vampire needs 3,500 calories a day to sustain his or her undeath (more than the average human adult, but it makes the math easier, and heck, a vampire doesn’t spend all night sitting at a desk), he or she will need to either rotate among six hundred cooperative hosts, or take down one victim every day. In the latter case, even if every victim is consumed after he or she reproduces, and even if vampirism is the only cause of human death, we would need a population of at least ten thousand humans to carry each vampire. By comparison, in the classic study of population dynamics among moose and wolves, the moose-to-wolf ratio ranges from 15–50 moose per wolf.
Moose… hmm. What if we are dealing with emo-pires who refuse, on principle, to feed off another sentient species? It says here that “a 400 kilogram moose has a blood volume of about 32 litres”, so one moose could replace between five and six human hosts; a moose generation is only four or five years, so a stable population of ten thousand humans could be replaced by about a hundred and fifty moose—assuming, crucially, that the vampires would be just as successful at catching the moose as they would be at catching two-legged prey. So Maine’s population of 30,000 moose (according to Wikipedia) could support up to two hundred vampires, while its human population could support only a hundred and thirty.
Regardless of which scenario you choose, given how much impact a single additional vampire has on the food supply, it’s hard to see why any immortal vampire would deliberately turn a human.
If vampire-story authors would pay attention to these questions of population dynamics, they could enrich the genre.