Scripting :)

Okay... how would I limit a number of selections in a field?

I got the limit of records in an application but having difficulty figuring out how to script a limit to a certain selection:

If I am running a clinic and have multiple clinics to chose from in a drop-down menu, how do I limit the number of applicants to one of those choices in the drop down?

Learning to script... slowly

Thanks for any help