Is if the maximum ads a user can have active? If so, does deleting an add free up a space?
Is if the maximum ads a user can have over a given time period?
Is it the maximum ads a user can have cumulatively?
The scenario that I'm investigating is that I'd like to give usergroup X, some number of free ads per year and then allow them to buy them as necessary.
Yes your assumption would be correct that on ad 6 they would need to pay if you have amounts set to pay for that category they are posting in sure.
sendnotice handles the expiration of ads because users have time to renew ads that are going to expire which that script sends emails for and expired ads it expires. Now once you go to your admin panel ads expired over 2 weeks are deleted in the background.