Its a chicken and egg problem - isn't it? How many categories should I create for my forum to not look too empty - How many will be too much for early adopters? I'm also struggling to understand how to get the ball rolling? I can reach out to groups and people may join, but how to get them to use my forum and start conversations? Any help or pointers to reading material (about starting and running a forum) will be gratefully received.
The more the merrier. Look at this forum, for example, it has a lot of cats and subcats yet it feels right. Forums are extremely hard to get going. If I was your friend and you wanted my honest opinion I'd say "don't do it". It will leave you disappointed.
I recently joined a new forum and after a week was offered a mod position. It's a French language site with a very cool format. Almost instantly there were about 50 members that joined up. For about a month things were humming along smoothly then it all dropped off seemingly at once. Now even the administrator doesn't log on. The place has been a cemetery for about 3 weeks. I do believe that it will go belly up soon as no advertisers have shown any interest in it. Getting the "ball rolling" as it were may not be the problem, but rather how to keep it rolling once it is. PM me if you like and I will check it out.
I'd go with fewer categories and be prepared to commit to creating more and moving threads about once you get lively. Forums are all but dead, the few that survive have a very specific mandate, tons of content, and often a "personality" driving them. Think very carefully about why you want a forum over, say, a facebook group.
There's a fair bit of discussion on this topic over on the SMF forums, so I hope I'm not out of line pointing you there: https://www.simplemachines.org/community/index.php?board=116.0 I draw your attention to this specifically: https://www.simplemachines.org/community/index.php?topic=551009.0 and my comments therein (same username/avatar so easy to spot) Best of luck.