Not sure... but people DO run into issues when they have around 1500 books on the Kobo. The kobo creates 5 images per book in the .kobo/images directory, not all of which are needed. I believe at least 2 images can be dropped per book.
Take a look for instance at
https://www.mobileread.com/forums/sho...d.php?t=170236 for details.