I was just trying to register my story for an ISBN number and the paperback issue isn't that much of a problem, but there's, like, a few thousand versions of ebooks. Epub with DRM, Epub Adobe with DRM, no DRM, Amazon Kindle format (with and without DRM or watermarks), there's no end to these things. I'm planning to cast my net as wide as possible, but I thought there were just three main formats. Epub, Mobi, and PDF, right? I assume that's three ISBNs right there, but I don't need every variation on a format, do I?
My [limited] understanding is that you don't need an ISBN for e-books at all. I chose to assign ISBNs to the Kindle and Epub versions of my books because the director of my local library informed me that libraries generally won't deal with e-books that don't have ISBNs. Going beyond that -- DRM or no DRM doesn't create another version of the work, it's a technical/digital copy protection technology that's intended to prevent unauthorized copying (piracy) of the work. From what I understand, different publishers/platforms may employ different technological schemes to implement DRM, which can prevent an EPUB book from Barnes & Noble Press (for example) from being opened on a Kobo reader device, but the underlying book hasn't changed, so I do not think DRM affects the assignment of an ISBN. Curiously, my previous, random readings on the question of DRM or no DRM generally seemed to suggest that I should not activate DRM. In doing a bit of research to inform my response to this question, the sources I found strongly suggest using DRM. Conflicting views: Pro: https://kitaboo.com/what-is-drm-digital-rights-management-and-why-you-need-it/ Con: https://justpublishingadvice.com/drm-is-the-biggest-downside-for-ebooks/
