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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.