Summary: It is believed by researchers of the English language that ordinary people — people without a college education — would know close to 35,000 words. A person who has gone through and completed a university education may know and recognize between 50,000–75,000 words quite easily.
According to the same source, the English language supposedly contains the most amount of words of any other, with the Oxford Dictionary saying that the total could be between one and two million, with certainty.