large in number or amount: many, much, great
This is the everyday Greek word for a lot of something. Its sense bends to whatever it is measuring. With things you can count, it means many or numerous (many people, many days, many signs); with things you measure by mass or by degree, it means much, great, or abundant (much fruit, great reward, deep grief). The shared idea is simple and constant: there is a large quantity of whatever is in view.
The word shifts shape easily. Used on its own it can stand for the people themselves, the many or the crowd, and in certain forms it slides into an adverb meaning much, often, or greatly. None of these are separate words; they are the same idea of largeness pointed at different things, so the writers move between them without strain.
A handful of New Testament uses carry real theological weight precisely because the word is so plain. When Jesus gives his life a ransom for many, and when Paul says that through one man's obedience the many are made righteous, the many is this same ordinary word. Whether it means a great number set against the few, or the whole company contrasted with the one, is a genuine question that the word itself does not settle.