>Merriam-Webster’s Third New International Dictionary lists about 470,000 words while the Oxford English Dictionary has some 300,000 words listed of which 47,000 are obsolete. Recently, the Global Language Monitor, the size of the English language crossed the 1,000,000. Clearly, the number we are discussing is very elastic.
With a vocabulary of 15,000 words you should be able to read about 98% of texts of which headwords account for around 72%. A native English speaker, for example, understands approximately 20,000 words by the time he/she finishes college.
The number of words you need varies according to individual needs: are you a banker, bus driver, academic, business person, entrepreneur, etc. As a speaker of English as second language a vocabulary of around 3000 high frequency words can be enough to get by.