I was putting away the books my DC got for their birthdays (a couple of months ago!) and took off the shelf some that were too young or had been read but I didn't think they would read again etc.
I can't help but think they have tonnes of books! I remember as a child having probably less than a quarter of what they have and read them over and over. Not sure they have ever re-read a book! They have 2 shelves each packed with books plus another 20 odd books on another shelf (tall ones that don't fit in bookcase). My DS also has a couple of big Trofast bins filled with books he has bought (as he is very precious about his books and doesn't like anyone touching them!) And that's only fiction they have a shelf each of non-fiction downstairs! My DD also reads books on her Kindle and has all the Mallory Towers, all the Worst Witch and various others on there.
My DS is a book worm and DD is getting really into reading now too. They are 14 and 9. I know some people say 'you can never have enough books' - I don't really agree with this - I only have a small house so there has to be some limits! And the library is always there.
Interested to hear how many books your DC have and if you set any limits?