You can use the newly registered users group for that. You can set it to no access for the other forums and then they'll only be able to see the one with access for the number of posts set in user registration settings > new member post limit.
However, you could also use the auto groups extension
https://www.phpbb.com/customise/db/exte ... to_groups/
You can set membership for groups to certain post numbers. Then you can give a new group access to what you want and have the membership set to X number of posts and the rest will be automated,