Can I set up a product limit?

how do i add limits to products i selling? 

Using the payment integrations it's not possible to set up limits to the products, but you may alternatively use a widget, this one will allow you to set limits: https://widgets.jotform.com/app/inventory 

The following guides will help you performing calculations as well and passing the amount 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.