The exact number of english words is not known. the large dictionaries have over half a million entries, but many of these are compound words (schoolroom, sugar bowl) or different derivatives of the same word (rare—rarely, rarefy), and a good many are obsolete words to help us read older literature. dictionaries do not attempt to cover completely words that we can draon: the informal vocabulary, especially slang, localism, the terms of various occupations and professions; words use only occasionally by scientists and specialists in many fields; foreign words borrowed for use in english; or many new words or new senses of words that come into use every year and that may or may not be used long enough to warrant being included. it would be conservative to say that there are over a million english words that any of us might meet in our listening and reading and that we may draw on in our speaking and writing.
Professor seashore concluded that first graders enter school with at least 2,000 words and add 5,000 each year so that they leave high school with at least 80,000. these figures are for recognition vocabulary, the words we understand when we read or hear them. our active vocabulary, the words we use in speaking and writing, is considerably smaller.
you cannot always produce a word exactly when you want it. but consciously using the words you recognize in reading will help get them into your active vocabulary. occasionally in your reading pay particular attention to these words, especially when the subject is one that you might well write or talk about. underline or make a list of words that you feel a need for and look up the less familiar ones in a dictionary. and then before very long find a way to use some of them.
once you know how they are pronounced and what they stand for, you can safely use them.