@Waylander: Well it all depends on type of books, or to be more precise how many of hight res images it countaing and/or embedded fonts. Text-only books are minuscule and you can have thousand of books like that, but "only" 100 of books (still more than enough IMO) like
this one (check the "File size" info
), and that is a fiction book, nothing fancy scientific or the like.
But that books uses high-res images for chapter headings +
those illustrations all of which look just as good on my new Paperwhite 3 with it's 300dpi screen as they did on my old Paperwhite 1.
As for how many books are on my Kindle. At the moment only 42 books, since I bought new Kindle 5 months ago and I'm still to lazy to transfer already read books from my old Kindle. I mostly read books that are part of series and keep the read ones on my Kindle until I've finished reading the series. And many, many series have more books yet to be writen and released then previous books are sitting on my Kindle for years, for me to be able to do quick search for character names and refresh my memory when new book in series finally comes out
Other then those already read books I rarely have more than 20 books on my Kindle waiting to be read, and usually transfer new ones in batches of 3-5 books at a time.