If you follow the current timeline, it should go-
Hurt->Eccleston->Tennant->Tennant->Smith->Capaldi->Whitaker->Tennant->Tennant/Gatwa
Hurt was the real 9th doctor, but we call Eccleston the 9th doctor. And (if I get this all right) Tennant regenerated as himself twice, so he should be the 11th, 12th, 15th and Doctor 16 and a half. But people call him the 10th doctor. And Jodie Whitaker should have been the 14th Doctor, but people call her the 13th Doctor. And then Tennant came back and then there was the bigeneration of him and Gatwa.
And just to add to it all, there's also a female clone of Tennant out there somewhere.
Is anyone else as confused as I am?
EDIT: Oh god, I just realized it gets even more confusing because now there's all the Doctors before Hartnell.
@CrabAndBroom @FlyingSquid There's also the Valeyard, which fits in somewhere between the 12th and nth Doctor, as well as Arabella Weir's alternate Doctor, Richard E. Grant's Doctor (who might be an alternate universe Doctor or a future incarnation).
The Children in Need skit can be ignored - as much as I adore Rowan Atkinson in the role - as well as the stage version. And Mark Gatiss, as great as he would be in a legitimate story.