Form to fill limited Volunteer positions

We'll be recruiting 350+ volunteers for 40 positions. The form will collect the basic name, address, email, etc plus volunteers will pick what position/shift they want to help. Is there a way to limit the number of volunteers submitting for a specific position. Example, we need 20 volunteers for registration....after we have 20, we either need to remove that option or show it filled. How can we limit specific positions?

Thanks

Doug