How to add Category Extensions and how many ppl does my game need to have in order to have a CE

https://www.speedrun.com/support/learn/requesting-a-game

"Each game is generally limited to one board. A "Category Extensions" board may be allowed to popular games to accommodate additional category sections on a case by case basis. Category extension boards must be requested by moderators of the existing game through the Support Hub."

tl;dr: Submit a request for it if you think it makes sense (assuming you're a moderator for that game), but it'll probably only be accepted if there is a clear need and the game is very popular.