If you are going to sell digital books, you need to support both major formats, Kindle (mobi) and ePub (everything else). However, the DRM only supports a number of simultaneous users, not a finite number of users, for Kindle. I don't knoe how the Adobe DRM for ePub works. Plus, you'd have to contract with Adobe to use their DRM, unless you sell through Kobo or something. And, of course, to use Amazon DRM, you have to sell through Amazon, and I don't know how that could work with a variable number of licenses. Maybe sell a different ASIN for different number of licenses?
No one in the world of ebooks is going to buy a book in some strange format.
It doesn't sound to me as if your sales model is very well-suited to the world of ebooks.
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