Yes dear readers, that's right, some clever clogs (one Brian Thomas*, in fact) has taken the maths and science applied to analysing population sizes and changes and applied it to the vampires of the Buffyverse, ultimately calculating exactly how many there'd have to be in order to have a stable ecology (i.e. not die out and also not have a population explosion).
If you don't want to read the whole paper, there's some neat summaries at these two sites that link to the piece; the latter even throwing in a little dig at Twilight for good measure.
This is far from the first attempt to 'study' vampire populations, I know, but this is a fairly decent - and, hey, Buffy-centric - one.
Quite understandably, the study restricts itself to Sunnydale. No doubt at least partly because there was some much crazier stuff going on with vampire populations over in LA...
...and don't even get me started on the whole "the grandaughter will sire the grandmother" thing.
*I'm such a Buffy geek. I initially read this guy's name as Brian Thompson.