How many words does the average person know?

Tests on about 220,000 people found the average 20-year-old native English speaker knows roughly 42,000 lemmas (dictionary headwords), rising to about 48,000 by age 60. That works out to nearly two new words a day through childhood and school, then about one every two days for the rest of life.

Source: Brysbaert et al. (2016), Frontiers in Psychology