This article on How Many Words Did Shakespeare Know? seems to be from 2002, but it’s making the rounds on all the bookmarking sites lately so somebody must have dug it up. In short, Shakespeare used about 31k words and we predict he “knew but didn’t use” another 35k, for a total of 66,000ish words.
What that *means*, I have no idea. I could not for the life of me give you even an order of magnitude guess at the number of words I know. What does it mean to know a word? If I’ve never used the word myself, but saw it written down and could figure out what it meant, do I know it now? Did I know it before then, too? You can’t exactly just start at the beginning and write down all the words you know.