Let's assume the following:

• Sunnydale's human population growth rate is 10% annually, which is at the high end for a budding California community.
• A vampire feeds every three days, and encounters about one hundred potential victims in the course of a day, meaning that 1 out of every 300 encounters involves a little refreshment.
• An individual vampire sires a victim every other year, or once per 240 feedings.
• Buffy and her Slayerettes, busy little beavers that they are, annually stake about 1/3 of the vampires plaguing Sunnydale.
• Vampires are flocking to Sunnydale, since the Hellmouth is the underwordly equivalent of Silicon Valley, and the demon labor market is just too good to be true. Thus, we'll assume a yearly migration rate of about 10%, or the same as for the humans.

[... The math starts off only slightly intimidating, but then gets downright scary. Don't skip out before the surprise attractor at the end ...]