It's very hard to put a figure on because it';s so hard to define- what exactly is knowing a word? Must I use it regularly, or is being aware of it enough? Would I have to know how to spell it? Indeed, how do we test to see how many words are in a person's head? Then there's the problem of what is a word- how many words is something like set, with dozens of different meanings? You'll see figures around the 4/5,000 mark, but this is usually the vocabulary used in the normal of events, without the passive vocabulary that is hard to measure. At the upper end, you'll see figures of 30,000 for Shakespeare, but he probably used more words in his writing than in his everyday chatter.
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