Many is used only with the plural of countable nouns (except in the combination many a). Its counterpart used with uncountable nouns is much. Many and much merge in the comparative and superlative forms, which are more and most for both determiners.
You use many to indicate that you are talking about a large number of people or things. I don't think many people would argue with that. Not many films are made in Finland. Do you keep many books and papers and memorabilia?
Definition of many determiner in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more.
Many can be traced back to the Welsh word mynych meaning "frequent." To get many of something, you do something frequently, or often, like having many miles on a car because you often drive it long distances.
A large number of persons or things: "For many are called, but few are chosen" (Matthew 22:14).