How Many Bitcoin Clients are there?
Are you asking how many different clients exist, ,or how many of those clients are connected to the network at any particular moment in time, or how many copies of the clients have been downloaded, or how many clients have ever connected to the network, or how many individuals might be actively in control of some bitcoin, or how many individuals are likely to have ever been in control of any bitcoin, or how many addresses have any bitcoin assigned to them at any particular moment in time?