I would usually aim for around 12 forums starting out. My ideal layout would be 4 categories with 3 forums in each category. That includes having the traditional category with announcements/intros/etc and the traditional off-topic category.
It’s difficult to say when it becomes too many nowadays, as so many forums have moved away from the traditional forum view layout.
I'm always keen to add many categories and boards.
But I end up trying to add less and combine more than to add more.
I usually stick with a few cornerstone content niche sections, an off-topic area, introductions, forum announcements, and a helpdesk for the public forums.
Then I try not to add anymore until it's really being demanded.
Honestly I know I need to condense the subforums I have over on Thee Zone, I have way too many. I also believe it depends on the niche of your forum as well. It's probably best to try and condense it down to around 12 at max.