I saw an article yesterday that said that the average English-speaking adult knows over 42,000 words.
That number was based on a study by Ghent University. They used an exam that they placed on Facebook. It took me a while to find it, but you can take it yourself HERE.
If you didn't take the test - and you should - it uses a very clever method of determining knowledge. It asks you if you recognize a series of words. Most are real, but about 30% look like they might be real words but are not.
Several of those fake words got me - and they warn you ahead of the test that there are extra penalties for saying you recognize a word that doesn't exist. In spite of scoring 96% on the real words, I lost major points by saying yes to predicity, doodlet and toaked.