How can I add a Quantity Option to a new product in a paypal payment integration?

To add a "quantity" to a new or existing product simply hover over the product field which you added then choose "add new option" then you'll see a new window pop-up with the first option already selected to add a quantity option  or textbox if needed.

Please see https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form for a better understanding of adding the extra options to your payment form.