An estimate of how many English language words does the average person know/use is typically in the realm of 12,000 - 20,000 words, varying with the level of education achieved. Some say college/university graduates use upwards of 20-25,000 words. These estimates apply to natives of the English language.
Shakespeare actively used more than 30,000 words in his written works, and his entire vocabularly has been estimated at approximately 66,000 words.
Some accounts tally the English language consists of more than 600,000 words (source: The Unabridged Oxford English Dictionary), and this includes root words and derivatives.