Among the famous writers that I know through their writings, I'd expect Jorge Luis Borges and Umberto Eco to have read many thousands of books, because that wide culture was a major element of their fictions. The later did have 50,000 books at home, but the former became blind and probably didn't own many books.

With simple math, reading 2 books each week leads to at most 7,000 over a lifetime. If Denny McCarthy's guess is right (read 85% of 20k), his older brother read about 4 to 5 books a week, every week, from teenager to his old age.