From Bleeding Cool:
...the Doctor isn't actually numbered that way for any in-universe reason at all. (Has anyone ever actually referred to him as "the [N]th Doctor" in dialogue on the show at any point? I can't think of one, but then again there are pretty big swaths of TOS I haven't seen.) He's called that as a reference to the actor who plays him.
And in that sense, yes, John Hurt played the Doctor for a single episode, but here in real life that doesn't make him the Ninth Doctor. Especially inasmuch as, oh hey, if you counted him in real life he would come AFTER Matt Smith. Because in real life, no, John Hurt did not actually play the D