Can a rule or something change the quantity each time a class is purchased?

is there anyway to shows the available quantity? Can a rule or something change the quantity each time a class is purchased since I only have 8 available seats per class?

This option is not available in the payment field or payment integration. However, you can use these widgets:

Inventory widget

Ticket Purchases widget

Gift Registry widget

Quantity Gift Registry widget

Here's a guide on How-to-Add-a-Widget-to-Your-Form. You can use the Form Calculation widget to get the total value of the selected options. You can refer to this guide: How-to-Perform-Form-Calculation-Using-a-Widget.

After this, you can just pass the value of the Form Calculation widget to the payment field. Please check this guide: How-to-Pass-a-Calculation-to-a-Payment-Field.

Hope that helps. If you have any questions, let us know. Thank you.