Can I apply a coupon based upon the number of items purchased, e.g. give a 15% discount if a customer purchases 10 or more items.

The coupon codes feature will only allow you to set discounts to products or to the total amount, but it is not possible to apply coupon codes based on quantities. 

You will need to set up your products using basic fields instead and then perform the calculations needed to get the total amount, this way you could alter the process and set conditions to apply the discount. 

Here is a guide that will help you performing calculations on your form: How-to-Perform-Form-Calculation-Using-a-Widget

This guide will help you with the conditionally calculated values: https://www.jotform.com/help/268-Conditional-Logic-Inserts-Text-Calculation-Into-A-Field 

And this one will help passing the calculation to the payment field: https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

Hope this helps.