Can't be easy to please everyone...
I have mainly ePubs, and I figure they are on average around 0,5 MB in size for a novel. Usually less, actually. Assuming that I have 20 GB free on my tablet, that is room for around 40 000 ePubs. Some of my books are bigger, especially nonfiction PDFs. Still, that will allow me to store at least 5 000 - 10 000 books on my tablet. That is many more ebooks than I have. So for now I store every (good/interesting) ebook I have on my tablet. The number of books slowly increase as I get/buy more books and fix up old books with bad metadata or bad covers. But most likely I will have room for every one of my (good) ebooks, on my tablet, for the forseeable future. This is great when traveling, since I don't have to decide what reading material to bring. I always bring everything.
Long before my current tablet is full I will most likely have bought a new and better tablet with more storage room.
Currently I use an external 32 GB external memory card to hold the ebooks on my tablet. That allows me to sync from Calibre using a folder as a device. I would like to be able to instead sync the files on my tablet, using CC over WiFi, after any changes or additions. It would be more convenient.
On my 32 GB removable card I have room for at least 64 000 novels in ePub format. I actually only use around 5 GB for books at the moment. Both PDFs and ePubs. Gives me plenty of room for some music and audiobooks and the odd movie.
On my computer I have my main Calibre Library, and it is there I fiddle with metadata and so on. I also have a few generations of backups on a NAS in the cellar.