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.
In your form in integrated to our Payment Tools, you can add a quantity option by the following the steps from this guide:
http://www.jotform.com/help/225-How-To-Add-Options-In-The-Payment-Form (I. Add Pre-Defined Quantity Property).
But I see that your form is configured to collect payments from individual textbox/calculation fields so, in this case, you will need to setup the quantity using Dropdown field and use a formula in the calculation.