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There is no direct way to do this on the payment tools as it does not have some type of inventory management system on it.
A work around I can think of at the moment is like this
1. Enable the form Submision Limit (guide: -How-to-Limit-Submissions-in-a-Form) i.e. set it to 20
BUT this can only work fully if you only sell 1 item per submission. Otherwise, if you sell more than 1 per transaction, the sold count will not match the submission limit anymore.
2. You can also make use of the Gift Registry widget to give visual cue on the form user of how many inventory left on the items. BUT take note that the counter in the Gift widget is not link in anyway to the payment tool.
You can just add instruction within the form to suggest to the user to use the same quantity used in the Gift widget to the item they are trying to buy. This could help you monitor then how many items left in the form notification e-mail.
Hope this help. Inform us if there is more we can help you with.