Yes it’s possible to have hundreds of subforums.
Working with your existing user accounts is something you will need to work out. It’s been done before though, just depends on how you keep your user information.
Let me see if I get this right. I can have automatic subforums for every movie I add to the database?
I don’t know that there is an automatic way to do it, but you can manually add subforums. As many as you need.
Where did the automatic part come from?
I imagine you could create a script to create a forum automatically when there is a new movie posted, but this is not something that is built in to bbPress? This would be some custom stuff you’d need to do. It would be pretty neat.
For the older versions of bbPress, there was bbSync https://bbpress.org/plugins/topic/bbsync/ which would create a new topic in the forum for every WordPress post (I believe that’s how it worked.) You could do something similar (but create new forums for every new movie.