You use many in expressions such as `not many', `not very many', and `too many' when replying to questions about numbers of things or people. `How many of the songs that dealt with this theme became hit songs?' --- `Not very many.' How many years is it since we've seen each other? Too many, anyway
You use many after `how' to ask questions about numbers or quantities. You use many after `how' in reported clauses to talk about numbers or quantities. How many years have you been here? No-one knows how many people have been killed since the war began. Many is also a pronoun. How many do you smoke a day?
emphasis You use many followed by `a' and a noun to emphasize that there are a lot of people or things involved in something. Many a mother tries to act out her unrealized dreams through her daughter
You use many to mean `many people'. Iris Murdoch was regarded by many as a supremely good and serious writer
emphasis You use a good many or a great many to emphasize that you are referring to a large number of things or people. We've both had a good many beers For a great many men and women, romance can be a most important part of marriage
The many means a large group of people, especially the ordinary people in society, considered as separate from a particular small group. The printing press gave power to a few to change the world for the many