Please create a function were a limit of payments can be added to projects like camps were only a certain amount of people can enter

Please create a function were a limit of payments can be added to projects like camps were only a certain amount of people can enter

I have an outdoor group and all my camps and daytrips can only take 13 children. I would really make more use of Sticitt for payments if my kids' parents can pay using Sticitt but the project closes the moment that 13 spots is paid for. I have about 90 kids in my club and only the first 13 that pay can go on either the camps or daytrips. A function like this will really help me a lot and I think lots of other schools who can only take a number of kids on camps, trips, etc. After the last spot is paid for the project stops on its own and blocks all further payments. 

I have about 6 camps in the year and almost 4 day trips during each term and this function would really help a lot and make me use Sticitt a lot more. Other sport groups would also be able to make use of such a function.