I always figured that vampires had to drink blood incredibly regularly, at least every couple of days, if not daily.
If we assume that each season of Buffy is the equivalent of about a year we can assume that Angelus was drinking blood for 6 months, that's what? 90-180 people that he killed.
Then again, to completely disagree with my own statement, if all vampires were draining that amount of humans we would barely have a human population!