It's not really a technical problem. Bandwidth for text messages is negligible and so is DB storage. The limited resource here is members and their available free time.
Let's say you have 1000 participating regular members. If you have 20 forums, then you might have an average of 50 participants in each. If you have 100 forums, then the average goes to 10 each... at one point, you "dilute" your base enough that most forums lose what little participation there is.
Of course, that analysis is very crude. Not everyone will participate in the same number of forums; nor will all forums be equally popular. But it still remains that if I, for example, spent time in "Computers/Video Games" and "Puzzles/Cryptography," then I'd have less time for "Politics" or other forums.
If anything, there are probably too many forums already.