Matt Smith is the 12th Doctor, or at least the 12th regeneration.
David Tennant cheating death, blow a regeneration, making his Doctor both the 10th and 11th regeneration of the Doctor.
I am well aware this view is not shared by fandom at large, but its my view.
The joke is in being able to substitute actor's names in for the positions of the clock, not
whether Matt's is the eleventh or twelfth Doctor. He's the eleventh actor
in the role.