An indexing sub-routine once said that this site had a vocabulary of a mere 12,149 distinct word forms (excluding non-indexed words like ‘a’, ‘the’, ‘and’, ‘an’, etc.).
This is rather less than a fully educated vocabulary!
In one sample of university-educated native English speakers, they were found to have a ‘receptive’ vocabulary of from 13,200 to 20,700 base words or ‘word families’. A word family is a base word and its inflections and derivations. [Goulden, R., Nation P, & Read J.(1990). How large can a receptive vocabulary be? Applied Linguistics, 11(4), 1990, pp. 341-363. Receptive vocabulary is words understood while listening or reading, which is usually large than one’s productive vocabulary, the words one is able to use to speak or write. There are other similar estimates.
Michael McCarthy, in Assessing development of advanced proficiency through learner corpora (Position paper. Center for Advanced Language Proficiency Education and Research, Pennsylvania State University, October 2007) sumarises research that shows that round 2,000 word-forms are used very frequently in everyday spoken and written English. Remaining, non-core, vocabulary occurs with relatively low frequency but is massive in size (between 30-50,000 word-forms being in use in everyday talk, and considerably more in everyday written texts, perhaps up to 80,000 words. To appreciate most texts, with an at least 98% recognition of words, a receptive vocabulary of much more than 10,000 words would be required.
Only very rarely do I encounter a non-technical word that I don’t understand. I would suppose my vocabulary to be up around the 80,000+ mark. Maybe I should inflict more words on you, dear reader.