Limit quantities to be sold per product type in Paypal integration.

I would like to limit the quantity of each product sold on our form.  

It's not possible to do that with the payment integration, but you could use a widget instead, for example, the Inventory widget could help you limiting the options: https://widgets.jotform.com/widget/inventory 

Then you will need to perform the calculations and pass the result to the payment field: 

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

I hope  this helps.