[Quoth]

I've about 3,700 books on my Sage now. Some are large PDF scans, but page size OK for 8" screen.
22.1 GByte of 32 GByte available.
At first use before adding books it was 28.4 GByte free, so that's 6.3 Gbyte of books.
My Original Libra had 6.7 GByte free of 8 GByte before books added. So it could certainly manage more than 4000 books if the scanned PDFs left off and only smaller page size manuals on it.

I don't see any reason not to have all the books (most are from Gutenberg which is why I bought an eink ereader originally) on the ereader that are on the laptop. It means any time of day or night, without electricity, mobile or laptop I have a library.

The Lady here is loving her Libra compared to the Kindle Paperwhite 3. She says she can so easily find any title or author. She found Kindle awkward to find books. Also Collections & Series views work properly from Calibre data. So she too is happy to have all her ebooks on the ereader.

We also have 4 off solar charged powerbanks and 2 off battery only power banks. So not dependent on electricity supply!