I'm not making fun (and I'm really not looking for an explanation either), but I really don't understand the desire to keep every ebook you've every read (or plan to read) on the Kindle. It baffles the bajeebus out of me... but I'm used to being baffled by others. For me -- Calibre is the library and the Kindle is the small book-bag.
Oh yea, the 3500 number?? All depends on the size of the books, the formats of the books, the number of illustrations in the books... etc. Basically the number is a WAG.