Custom Field - setting registrant limits

Is there any documentation on how to set field limits.  I have an event reg form build but there are 2 sessions and i created a custom field drop down menu for the AM session or the PM session. We only want to allow 10 registrants for the AM and 10 for the PM session.  
I filled out the QTY values in the custom field and registered 10 people on the backend for the AM session and it still allowed me to select the AM session. I didn't actually try to submit the form but just wanted to know how that all worked.

Thanks

Hello Bay

1. When you using a super admin account, we do not perform quantity check, that's the reason you could select. Try to register on frontend with a standard registered user account (or not logged in), you should not be allowed

2. However, quantity field is not reliable and could cause overbooking (imagine there is one slot left for am option and two users register for it at the same time). Maybe you can use Ticket Types, it handle validation better. See eventbookingdoc.joomservices.com/plugins/ticket-types

Tuan