I understand that in free ads it asks for number of free ads before paying / but this is for total ads and once used it is used, but can i edit the system to allow the number given to represent the number of free ads at any one time not total time.

for example if I set it to say 10 then it would allow them to have up to 10 free ads at any one time, from there they would need to go paid / or if they wanted to they could delete one of the free ads and then add another free ad, and they could do this forever and a day so long as they never exceeded the free allocations set.