If you have a great number of refrigerator magnets, you have many of them. Many means "a lot of items."
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. If you ask, "How many people will be at the party?" you want to know the number of guests who are expected. If the answer is "many," you know there will be numerous people, though you haven't been given an exact figure.