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If I understood you correctly you want to allow your users to enter custom quantity for product they select.
You can achieve that by checking Custom Quantity (Text Box) options, when you are adding new options to your products.
Also inside of this guide you can find how to add options to Payment Forms:
Here is my demo form: https://form.jotform.com/53424771319962?
Hope this will help. Let us know if you need further assistance.