How many words are there in the English Language?

As the Oxford English Dictionary says, there is no sensible, single answer to this question. The problem lies in deciding exactly what a word is! Let me give you an example. The word 'dog'. Should it count as one or two words? A dog is an animal, that's to say a noun. But it is also a verb meaning to follow persistently. And what about 'dog-tired'? Is that another word or just two words joined together? What about French words used in cooking or Latin words used in law.

What about dialects, scientific terms, slang?

The Second Edition of the Oxford English Dictionary contains full entries for 171,476 words in current use, and 47,156 obsolete words. To this may be added around 9,500 derivative words included as subentries. Over half of these words are nouns, about a quarter adjectives, and about a seventh verbs; the rest is made up of interjections, conjunctions, prepositions, suffixes, etc. These figures take no account of entries with senses for different parts of speech (such as noun and adjective).

This suggests that there are at least a quarter of a million distinct English words, of which about 20 percent are no longer in current use.If distinct senses were counted, the total would probably approach three quarters of a million.