I think it is nearly impossible to know how many people in the whole world are using Bitcoin, and even you know how many Bitcoin address all in all out there it will not give you the accurate answer, because people can have multiple Bitcoin addresses, I myself also uses more than one address for storing and more than one in different exchanges. And also exchanges cannot give you an accurate answer because there are people who invested in Bitcoin by just holding it and decided not to do trading in any exchanges or trading platforms.