I believe a maximum of 1.2 million. Of course, many people have several addresses, and some have dozens of addresses. Even the average bitcoin user will probably have an address associated with an exchange, one that operates as a current account, one as cold storage, and so on, and so on. Not all of those will have coins in at any given time. Many have tiny amounts that the owner has forgotten about. So whilst its hard to say, Id estimate that between half and three quarters of a million people is reasonable ballpark range. Its worth remembering that this is a snapshot of the blockchain. There may be other people who have owned bitcoins in the last week or month, and can be classed as active bitcoiners, but who didnt own any right at that moment.