JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You may want to try the Gift Registry Widget.
This widget have a Quantity count that diminish or count down every time you select an option/item among the available options.
Here is a demo form you can test http://form.jotformpro.com/form/41825991521963
If you search the Form Widgets for 'counter' it will give you this result http://widgets.jotform.com/search/counter -- you can check also the other similar widgets.
Hope this help. Inform us if you need further assistance.