Exchanges are the same as auctions in the sense that the person that is buying the item is the one who bid the highest price. There is a reason why the buying prices is called a bid.

Auctions are not limited to a single item, radio frequencies and bus routes are just few of the examples that are sold in multiple item auctions. There is nothing special about the number one in auction rather than a single item is limited in supply, but in most cases two items can be of limited supply as well and even millions. Good example for that can be engineers. Even though there are many of them, their supply according to many is still limited, which causes their salaries to rise.