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There are two features that you can try for this.
1. Use form submission limit
You can assign a limited number of submissions the form can have before it becomes disabled.
The downside to this is, the diminishing of available submissions (proportion to available "seats" ) is only one at a time -- so the Quanity is not being considered anymore
2. Use the Gift Registry widget http://widgets.jotform.com/widget/gift_registry
Here is a sample form http://www.jotform.me/form/41307497219458 how the idea will be.
If you select the item "Seat to Paint", it will diminish by 1 every submission.
Hope this help. Inform us of your thought on this solution.