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Unfortunately, there is no built in feature to limit the use of coupon codes. A coupon code which is created can be used multiple times.
Currently, the only way to limit your form is by changing the preferences of "Form Limits". A possible workaround is that you create only one coupon code and set the form's "Submission Limit" to desired number. Here is guide to help you understand form limits: http://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form
There could be another possible workaround which will require adding custom script in your form's source code. This is just an idea and I am not sure if that can be implemented. If you are okay to use your form's source code, I can give it a try.
We will wait for your response.
I am happy inform you that it is now possible to limit coupon usage, either by expiration date, or by number of uses.
Please try it and let us know what you think.