The estimate is based on how many “stem” words one knows, so it doesn’t include, say “quick” and “quicken”. The average vocabulary may be anything from 5,000 to 45,000 words (including variations on a single word) according to researchers. The active vocabulary is, of course, much smaller. Indeed I read somewhere that one would need to know only about 1,600 words to read the New York Times. Take from that what you will!