One. I still get confused when installers etc ask me whether to install for "just me" or "anyone using this computer"!
Things get a little fuzzier on Linux machines where there is both my user and a an admin "user". I treat the other as an abstraction, and type "sudo" if it won't let me do something without invoking that. Sometimes there are problems where I need to install something with sudo, then can't access it as my normal user and vice versa.