What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
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.