Vocabulary counts are notoriously unreliable. How you decide whether a word is distinct from another can often be very tricky. (Is a computer “mouse” the same word as the rodent? What about inflections? Is “fun” both a noun and an adjective, or just a noun that sometimes acts adjectivally?)
But the common number trotted out is that the “average” (whatever that means) educated English-speaker knows about 20,000 words. There are, of course, some people who know twice or even three times this number of words. A pocket dictionary will contain something like 45,000 words. Merriam-Webster’s Pocket Dictionary has some 40,000 words.
But it’s not as simple as comparing the numbers. After including the few thousand most commonly used words, the lexicographers will tend to include words fall into several categories, such as 1) easily misspelled; 2) often misused; 3) difficult words but ones that are likely to be encountered in literature and newspapers, including common jargon of particular disciplines. And what words are included may change with the intended audience; a dictionary aimed at ESL students will cover a different range of words than a standard reference. So there’s likely to be less overlap than a simple count would indicate.