There are at least 70K public domain ebooks.

There are sources that say there are 1 M public domain ebooks.

https://libguides.library.umkc.edu/OER/PublicDomainBooks

Also there are thousands and thousands of DRM free ebooks to purchase. Possibly millions.

https://www.tomsguide.com/tablets/e-readers/no-kindle-no-pro...

Most ebooks can be found online drm-free extremely easily and/or you can remove the drm yourself.