Slient group Subscribe when sign up
Silent subscribe group number to post.. for example.
Globally - 1 for default 'Register' allow to post 1.
Different Register group - 5. so it detect to subscribe plan (silent) so it overridden global setting..
So it doesn't have to use subscribe visible. Unless you set visible subscribe plan.
I have tried to set it 'Free' via subscribe without doing anything (New Member) so it doesn't work.
You can achieve this with Subscription Plans, you can assign user to some group in administrator and then create plan that will exceed number of adverts and will be available only for suitable user group. This feature won't be visible in front page.