How to limit each class on my form to 50?

I have a Class registration form.

Each class hold 50 people.

I would like "Class Full" to pop up when someone tries to register for a class that already has 50 people signed up.

Thanks!

My form:

https://www.jotform.com/build/63534925595164#preview

I understand that you want to limit the form submissions to 50. If so, you may set the limit from the form preferences. Please refer to the guide below that can help you with limiting the submissions on the form.

https://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form

Hope this information helps! Please get back to us if you need any further assistance. We will be happy to help. 

Sorry for the misunderstanding. I checked your form again and see that you wanted to limit each class to 50 entries. I see that the classes are provided with a checkbox field on the form. In order to limit each class to the form, you may consider using Inventory widget so that it can also be used for calculation.

https://widgets.jotform.com/widget/inventory

Hope this information helps! Please get back to us if you need any further assistance. We will be happy to help. 

I will take a look. Thanks. Can I have a message appear when someone tries to register for a full class?

If you are using the inventory widget and when the limit is exhausted, the option to select the class will be disabled and displays as None available to select as shown below:

Hope this information helps!