I think with this one we have to assume that any body that can be thrown into Lake, will count towards the body count.
FIrstly, to be 100% sure you would need to throw 800-1000 of only one type of body (eg. skeleton) into the Lake and be sure no one else was throwing bodies in which would take a considerable and coordinated effort.
Secondly, the Programmers who set up this World Change system would most likely have set it so any 'body' thrown in would count, otherwise they would have to start doing what I imagine would be an unnecessarily boring task of individually marking each corpse as good or bad. But Im not a programmer so this ones way out of my wheel house.
But if you ever want to test this out, give me a couple of days warning and Ill spin up a character on your world and help you out