^But even if one regeneration is used up, Tennant was still only one Doctor, not two. So Smith is still the Eleventh Doctor.
And he's only the tenth regeneration, or eleventh under your scheme. People tend to overlook the "re-" part at the beginning. The First Doctor didn't regenerate into existence, he was just generated, i.e. born. (Or loomed, if you go by the Virgin novels' continuity.) The first regeneration produced the Second Doctor, the second regeneration produced the Third, and so on.
but we still have one less Doctor than we think we have, if we only count Tennants Doctor once.