Easy Digital Downloads is a wonderful plugin to use to sell digital products on your site. The plugin is powerful on its own, but you can always install additional extensions to do more with it. Purchase Limit is an EDD extension that lets you specify purchase limits for your products. It also lets you restrict purchases to a specific date range.
This extension lets you set purchase limit for individual products. You can also choose a limit for file downloads. You can change labels, start date, end date, and other settings from the backend. Webmasters can also choose the right scope for Purchase Limit.
Purchase Limit is a commercial extension. You can find out more about it here.