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In order for you to set a limit on the amount of products to sell, I would recommend you to use our Intventory Widge, do as follows:
1 - Open your form in edit mode, from the widget section, drag Inventory, the Inventory Settings screen will pop up, fill in the blanks, as shown below:
Notice that you are also able to include a picture to your product, click Finish button when done:
Please take a moment to read this guide: http://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form
I hope this information helps.