It is a delicate balance maintaining the right number of sub fora. One of my favorite a decade ago (electro-music.com) is still there but is very very dead, I think because they have something like 100 sub forums, so you would need a 1000 daily posting users to make it seem active. On the other hand, you have forums (gearspace.com) which don't seem to have enough subs so the main forums (I mostly go for the Electronic music forum) which makes it seem a little too busy or chaotic.