According to an article entitled The words in the mental cupboard published by the BBC, "An ordinary person, one who has not been to university say, would know about 35,000 quite easily."
The Unix dictionary contains far too many ridiculous words that even Google has
trouble explaining, such as zuurveldt, cholecystenterorrhaphy and nonly:
$ cat /usr/share/dict/words | wc -l
235886
Even enable1.txt, the more verbose version of the Official Scrabble Player's
Dictionary (ospd.txt) (which is limited to words of 8 letters or less) used
by Words with Friends, contains more words than any English speaking adult
would reasonably be familiar with:
$ cat enable1.txt | wc -l
172819
popular.txt represents the common subset of words found in both enable1.txt
and Wiktionary's word frequency lists, which are in turn compiled by
statistically analyzing a sample of 29 million words used in English TV and
movie scripts.
$ cat popular.txt | wc -l
25322
These are 25,322 words that everyone should be familiar with.