a great many vs many

a great many

det

  • A substantial number of 

many

det

  • An indefinite large number of. 

  • Used to indicate, demonstrate or compare the number of people or things. 

noun

  • A considerable number. 

  • A multitude; a great aggregate; a mass of people; the generality; the common herd. 

adj

  • Existing in large number; numerous. 

pron

  • An indefinite large number of people or things. 

How often have the words a great many and many occurred in a corpus of books? (source: Google Ngram Viewer )