I touched briefly on this in the very first blog post here at phpBB.com. If you’ve just set up phpBB, how many forums should you create?
As I said in that blog post, the key is to start small and increase the number of forums you have over time. There is nothing worse than going to a forum which takes a minute to load, takes twice that long to scroll to the bottom of the page and has dozens upon dozens of inactive forums with either a handful of posts or none at all. Even though there might be a couple of very active forums amongst it all, many people will still think your forum is inactive.
This is why it is better to have a couple of forums at first and expand over time. I think a good starting point is 3 forums – but this could differ for different boards – because users will have enough different areas to discuss unrelated topics, while also being small enough to ensure your forum looks active and allow users to feel comfortable that they are posting in the correct area (which could be a problem if you have a lot of different sections!).
Take for example a New York Yankees baseball community – you might want to create different forums for the pitchers, the batters, draft/trade talk, player development, rumours, etc, etc. You could go on to create a fair few different forums, but there is a risk that some might become very active and others might not. So instead, start with an Announcements forum (which most forums will have to serve as a place to inform users about changes to the actual website/forum) and a “Yankees Discussion” forum. Inside that discussion forum, any Yankees related posts can be made. You could also make an “Opposition Forum” to discuss other baseball teams or non-Yankees related topics.
By having a small number of forums it also decreases the chances of topics going unnoticed. If a user posts a topic then in most cases they want people to reply to it. If you have several dozen forums, then the chances are people are not looking at them all – so if a user posts a new topic in one of them, people may not see it and therefore won’t reply. If this happens enough it could drive existing users away. When you only have a few forums, chances are that people are constantly looking at them so the topics posted in them will receive more views (and more replies).
After a few months, you will have a general idea of what people like to talk about by reviewing the topics in the discussion forum. You can then expand the number of forums accordingly.
If unsure, in phpBB3 you can create sub-forums. You could use these to judge how popular a new forum might become if you don’t want it to appear on the index.
As a final note, there is no reason to worry about the number of “private” forums you have. If you have a couple of forums only accessible to administrators and moderators, that is fine. These forums do not need to be overly active because the general public will not be looking at them, so it won’t factor in when users are considering how active your site is.