How to create a field that increments based on criteria

I hope the subject didn't throw anybody off...

I'm working on a database which is used for a local summer camp. The camp
has a little snack bar which is run completely manual. Each camper is
assigned a unique number (for that week's camp) which the snack bar operator
can then use to access the camper's account (all done on paper).

The database is used to register campers, track finance and assign camp
cabins. We'd like to also assign the snack bar number automatically... but
we want the number to reset for each camp (ie - for each camp keep a running
number). The reason we don't want it to continue to run is so the kids can
more easily remember a low number (150 max). To make things more confusing -
we are using the database to register kids to multiple camps at a time. Is
there a way to do this - or do we need to continue manually assigning the
numbers?