MyBB Community Forums

Is there a plugin that restricts a user or all members from entering a specific forum until they have the needed amount of posts?

I am not really a MyBB Pro but I think you can try doing this

1. Make a new users group (Promoted Users)

2. Make a new Users Group Promotion with these settings,
|_ Make the settings (Post count > Greater than your desired no. of posts)
|_ Set the new user group to Promoted Users.

3. Now go to your FORUM setting and make it visible only to "Promoted Users"

I think this should work