Setting up a count to restrict registrations

I'm constructing a database for a summer camp and I need to be able to
allow only a certain number of male and female registrations per camp
session as well as only a certain number of waterskiing and
wakeboarding campers per session. Right now I have a form that uses a
dropdown to select a camp session and checkboxes for waterskiing and
wakeboarding. How can I implement the above limits to display a
msgbox() when a camp session is full or waterskiing or wakeboarding is
full for that session?

.