@terdon:
wikipedia.org/wiki/List_of_legally_mononymous_people lists 25 people, currently living, whose legal name is a mononym. And remember, Wikipedia requires notability, so there's probably plenty of people with mononyms that aren't notable enough to be listed on Wikipedia. For example, Turkey only established surnames after WW1, so there might still be some (very old) people alive that don't have one. Parts of India and several Native American and First Nations tribes use only one name. Mononyms are not part of US tradition, but it is legal to change your name to one. And so on …