I would think that if you were starting your own forum you might want to keep it to 1 forum, and let it grow from there, open up more forums as the needs arise.
I have always thought the less forums the better, it has worked well for gfy they have 5 forums which isnt a great amount but it has allowed them to get close to 10 million posts in the main forum, if it gets to a size were posts are getting buried within 10 minutes (like gfy) then maybe another forum is the way to go.....