How To Add permission On A Specific Forum That those user will post in this forum who have at-least 50 posts>>>
You can use the group promotions to set something like this up.
Create a new user group copying the Registered groups permissions. Then edit the original Registered group so they can't view said forum. Then set up a promotion from the Registered group to your new group at 50 posts.
But I Want These Permissions on Every Group?>>
then add the promotion to the new group for all of the other groups you want it to apply to. you can be in more than one additional group