I have 12 items listed on my order form. How can a customer order more that one of the same items?

FORM :  Spirit Wear Order Form

To do that, all that you should do is to add a quantity field to it.

You can see more about that here: How to add options to Payment Forms

Although just looking at your form here: http://www.jotform.us/Princville/spiritwear I see that you are already using the quantity fields on the second page.

Please do let us know if you happen to have any further questions and we would be happy to assist :)