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It seems like you have already figured out how to add a quantity option in your order form. I checked your form http://www.jotformpro.com/form/60966095682973.
To help you further on this feature, you can check this guide on how to add options (ex. quantity) in your product list: https://www.jotform.com/help/225-How-to-add-options-to-Payment-Forms.
Let us know if you need further assistance on this.