How can I limit the amount of items that can be purchased in my form?

If I have only 12 seats in a class, how do I have the form not receive anymore sales after 12 seats are sold?

You can limit the number of submissions in your form as described in this guide: https://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form

In your case, one submission will count as one seat. So, you need to limit the quantity of painters per purchase to one (if this is how you control the available seats) in order to make the limit works correctly.

Let us know if this helps.
Thanks.