The first time someone creates a forum they create a bunch of different sub forums because they think it will spark discussion. In my experience has the opposite effect, and dilutes the conversation.
I would suggest: Starting with ONE (maybe two) forum/s to begin with, and then adding more as the community grows and their need becomes obvious.
I think the bazillion individual forums or boards is why the Rifftrax community feature is so underused. It quickly grew into an exercise in organization which was a turn off to people who just wanted a quick chat.
I know this forum doesn’t have 20 sub-forums, but still I think that
Forums Feedback
Artsy Crafty/Show Us Your Bots
Cheesy Movies
Extended World of Riffing
Could all be really vibrant and exciting THREADS in MST3K Central
And then if they get too vibrant, then it would be a great excuse to take them into their own sub forums. The only real reason to create a separate sub-forum is when the contents of it would interfere with another (eg. “Off topic” makes sense to separate out).