I'm not sure if this is the place to post this, but I thought I'd try here. I posted it in the HTML forum, and I was told it would require a server-side language. In addition to the question below, I would also like to know: Which language, and how would I do it?

I am working on an application form for an online class, and they would like to limit the number of applicants to 20. Right now, there's only one class listed, although we're hoping to add multiple class listings at a later date to the same form (in addition to the checkbox at the bottom), so each class offered would need a limit of 20 applicants. Once the limit is reached, anyone trying to apply for that class would get a message that says "Class is full" (or automatically put a note next to that option when the limit is reached). Any ideas of how to do this?
The form is here: http://www.dualreach.org/missions/re...-101-reg.shtml