Permissions - How do I set up post count based membergroup permissions
From Online Manual
SMF allows the setting up of permissions for Post-Count based membergroups. When enabling this option, the permission for each member will vary depending on the user's number of posts. Keep in mind that post count groups also inherit permissions from the primary group that they are in. If they are not in a primary group they inherit permissions from the "Regular Members" group.
Once enabled, all the post count based membergroups will be listed along with the normal membergroups when going to Admin > Members > Permissions > General Permissions. Selecting Modify next to the membergroup of interest makes it possible to define specific permissions for each and every post count based membergroup.
Note that revoking a permission to a membergroup does not necessarily mean that members belonging to this membergroup will be unable to perform that action. See Disallow and Deny - What's the difference when it comes down to permissions for details.