Hey, the churches of Lathander and Kelemvor can always do with some more stuff to do.
But, on a more serious note. The number is just something Larian's writers pulled out of their butts that sounded imposing and dramatic. They obviously did not consider the sheer logistics behind such a population of vampire spawns. Even before releasing them, the entire premise of Cazador housing such a population in his dungeons is implausible. On top of the sheer size of that dungeon (especially if the cells where we see Sebastian are to be considered the standard), he would need a massive workforce of slaves and servants to maintain it and a constant traffic of supplies going to and from the palace.
So you need to use suspension of belief to accept that this spawn population exists in the first place, so what happens to them afterwards is simply a matter of applying more of that suspension of belief.
It's really one of those magic number games like Astarion's gravestone, where I can only wonder what the writers actually meant to imply. The state of their lodgings in the undercity, I generally blame on the whole structure belonging in the upper city and so being possibly largely unfinished.
