Project guidance for Volunteer Scheduling

Project guidance for Volunteer Scheduling

We'd like to create what should be a rather simple application - an online site for volunteers to signup for various available positions (for an annual fund raising cycling event). 

My stumbling block is how to limit the number of specific positions available to volunteers (e.g. there are 15 1pm-5pm food line workers...not 16).  How do I structure the database to limit the number of specific positions available?  It's not unlike students signing up for a limited number of seats in a class....the relationships (and reporting) are the same.  I'll have a Volunteer table, a Position table...but it's the structure I missing.

Thanks