Duke 3d's mapping limit is really big its over 16k walls and 16k sprites. Duke measures by how many walls, sectors, and sprites you have and there aren't many user maps that use that amount and none of the maps in the original game even come close to reaching this number. That Duke map took me over a year to make but since its still not finished I guess I can say over two years now hehe

I have a feeling Doom's is probably around the same but maybe a little more the limits on these things are ridiculously big and most likely won't be hit.
As far as trigger on death/ spawner this demon on my map(where the blue key is) triggers enemies to spawn. You can probably reference it for sure

Revenant is my favorite Demon and he is added into the Brutal Doom 64

But not Arch-vile.