The runtime has some limitations when it comes to the forum, they are defined here: https://github.com/Joystream/joystream/blob/master/runtime/src/lib.rs#L734

L1dev put together a nice list:

• You can have max 6 levels of categories
• Each category can have 20 subcategories
• Each category can have 20 threads
• Each thread can have 20 posts
• Each category can have 20 moderators
• There can ban 20 top-level categories in total
• There can be 20 possible answers in polls (not sure about that one)
• PostLifeTime: 3600 blocks (Maximum number of blocks before a post can be erased by anyone)