I think that “how many categories” is probably the wrong approach.
I think that you should consider creating categories that you think will get action and new posts every day (or perhaps) 3-4 times a week.
Forums with categories that have week-old, month-old and sometimes even year-old gaps between last post and current date are failing.
So if you’re starting a new forum – just create a handful of high level categories. If one category really takes off and starts getting to confusing then try a logical spilt.
I think this organic growth and tuning works better than attempting to be a “forum nostradamus” and predict where posts will occur.
:hattip: