How do you set a stock limit for each product?

The system only allow to set limit of submission to the whole form but we have to limit number of response for each category in the payment section.  Like for example, we can only accommodate 40 children for the registration. 

We do not have a built-in stock limit option, but we can use a combination of Inventory widget, Form Calculation widget and one of available payment tools. The idea is to calculate a Total amount and forward it to the payment field:

How to pass a calculation to a payment field

Please see the next video:

http://screencast.com/t/wvBMKrTNZKCy

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