Babi, I can only speak for Kindle, which is great for the eyes. There's no glare and you can adjust the size of the print, which I like.
I think the number of books is something like 3500, but that may be an exageration. Amazon can store them, but don't ask me how. How it works with other its, mobiles, tablets, etc, I don't know, but would guess it would be based on the units memory, or how the provider achives the titles.
There's a lot to keep up with in all this technology business, isn't there.