Does anyone know how many words an "average" literate person can use/recognize?

Does anyone know how many words an average person knows and uses? I have no idea but I figure it must be tens of thousands. Does anyone know or have heard a correct number? A source would be nice if you have one :-)
Update: I put the word average in quotations for a reason, meaning it is a very relative term. If you don't recognize that then you are probably in the "below average" category.
Update 2: Actually, literacy itself has a universal definition:

United Nations definition - the ability to read and write a simple sentence in any language.
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