How is the way I can create checkbox (e.g. for class enrolment) with the left of available number of seat?

The example is on the url below:

https://secure.jotform.me/form/52879417774472

To achieve your requirement, all you need to do is to add the Ticket Purchases widget to your form. This widget allows you to display the number of seats available for your class.
https://widgets.jotform.com/widget/ticket_purchases 

You can add a widget to your form by following this guide: How to Add a Widget to your Form?

If you need further assistance, please let us know.
Regards.