I have 3 different libraries, one for comics, one for TTRPG books, and my main one for actual eBooks. My main library is over 4GB with over 1,000 books.
No, I haven't read all of them, but I am working on it. They have been collected over the last 15 years from Sony eReader store, Kobo books, Amazon Kindle, B&N, Archive.org and other Public Domain books hosts, Humble Bundle, Story Bundle, and various other places.
Largest categories are Sci-Fi, Fantasy, and Mystery.
I have around 7,750 books in mine at the moment. A significant chunk of this was a collection of sf and classic novels that a friend gave to me about 12 years back - boosted by a couple of smaller collections later.
I also have around 2,000 physical books and am slowly thinning those our by adding ebook versions to Calibre.
As for an Android app, I use "Calibre Library" with mine - usually downloading through that and reading in ReadEra.
Looking it up I see that it hasn't been updated lately and only works on older versions of Android. I have a really old phone at the moment and am going to replace it imminently, so I'll be very sorry to loose that.
Wow, absolutely smashing me. I have just moved across to EBooks from physical books because of travel so i have very very minimal books atm, slowly buying them all.
I think at one point I had 14,000 books when I was a teenager (all pirated.) I've since deleted most of that and have sub 200. It's all in one library now but I used to have one for Textbooks/Nonfiction, Fiction and one for Cooking. I have been trying to do things more ethically this time around but I really don't like DRM because you don't really own the eBooks you purchase.
I have on library stored on my NAS drive. It contains magazines and for the most part TTRPG books. Only about 200 in total. I run a calibre-web in a docker container and connect thru Tailscale using the Firefox browser so that I can just read what book I need based on what game we play. The magazines I access in the same way but on my phone when travelling.
There's a sync app for Android just noticed the other day on pstore - just getting into calibre so bit too early for me to be using it. Looks cool though.