Such thing as a restrictive forum access?
 

Is there a hack where only users can get in if they have certain amount of posts?

Like I know you can have moderators only forum but lets say I want a forum with people more than 2000 posts can enter.

Create a new usergroup with the appropriate permission to access that forum and deny access to your regular member group.

Then use the usergroup promotion feature to add the new usergroup you created as a secondary when they hit the right post count.