This site on famous book hoarders implies that 1,000 books are enough. But Karl Lagerfeld--who woul have known?--owns 300,000 and Hannah Arendt 4,000.
I would have liked to know what kind of books Lagerfeld owns. I know the kind Hannah Arendt owned. My own "collection" is very close to Arendt's in number and kind.
Oh ... and the article has beautiful pictures of libraries.