Definition of many determiner from the Oxford Advanced American Dictionary

      

    many

     determiner,pronoun
    determiner, pronoun
    NAmE//ˈmɛni//
     
     
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  1. 1 used with plural nouns and verbs, especially in negative sentences or in more formal English, to mean “a large number of.” Also used in questions to ask about the size of a number, and with “as,” “so,” and “too.” We don't have very many copies left. You can't each have one. We haven't got many. Many people feel that the law should be changed. Many of those present disagreed. How many children do you have? There are too many mistakes in this essay. He made ten mistakes in as many (= in ten) lines. New drivers have twice as many accidents as experienced drivers. Don't take so many. I've known her for a great many (= very many) years. Even if one person is hurt that is one too many. It was one of my many mistakes. a many-headed monster
  2. 2 the many used with a plural verb to mean “most people” a government that improves conditions for the many
  3. 3many a (formal) used with a singular noun and verb to mean “a large number of” Many a good man has been destroyed by drinking.
  4. Idioms
    as many as…
     
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    used to show surprise that the number of people or things involved is so large There were as many as 200 people at the lecture.
    have had one too many(informal)
     
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    to be slightly drunk
    many's the…(formal)
     
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    used to show that something happens often Many's the time I heard her use those words.
Grammarmany / a lot of / lots ofMany is used only with countable nouns. It is used mainly in questions and negative sentences:Do you go to many concerts? How many people came to the meeting? I don’t go to many concerts.Although it is not common in statements, it is used after so, as, and too:You made too many mistakes.In statements, a lot of or lots of (informal) are much more common:I go to a lot of concerts. “How many CDs do you have?” “Lots!”However, they are not used with measurements of time or distance:I stayed in Ann Arbor for many/quite a few/ten weeks. I stayed in Ann Arbor a lot of weeks.When a lot of/lots of means “many,” it takes a plural verb:Lots of people like Italian food.You can also use plenty of (informal):Plenty of stores stay open late.These phrases can also be used in questions and negative sentences.A lot of/lots of is still felt to be informal, so, in formal writing, it is better to use many or a large number of in statements.
See the Oxford Advanced Learner's Dictionary entry: many