In the series, Buffy routinely goes out on patrol in graveyards, staking new vamplings as they rise from the grave. It always seemed to me like vampires are spontaneously generated from corpses via the Hellmouth - but this isn't the case at all.
From the Buffyverse wiki, vampires are created via Siring, a multistep processes that starts with the Sire draining the blood of the human until near death, than allowing the the human to consume their blood. They do not become a vampire immediately however, as Wesley states it takes until the following night for them to 'awaken' in vampire form (although apparently this isn't a strict rule and they can rise sooner).
The problem is the sheer number of vampires that Buffy is able to stake, night after night, must mean that new vampires are being sired constantly in Sunnydale. And they must be coming from other vampires. But the vampires shown in the series don't seem to be doing this - usually, they are more interested in killing victims outright then making new vampires.
Is there one vampire constantly siring new ones for Buffy to kill that she never manages to catch? Are new vampires siring more new vampires as soon as they awaken to create exponential population growth?
If Buffy is doing her job, eventually she would just run out of new vampires, as she'll have killed anyone left to sire them - save for vampires that moved in from other towns that she might not know about, or older established vampires, etc.
Has there ever been an official explanation as to where are all the new vampires that Buffy kills every night are coming from?