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.
He’s number 15.
The War Doctor wasn’t the Doctor because he abandoned the promise he made and the name along with it. So, Tennant was the 10th. He didn’t fully regenerate that other time in the Meta-Crisis, so he was only/still 10. The Other Tennant is a human, not a Time Lord, and so not the Doctor. Plus, he lives in another universe. The Doctor’s Daughter was also a clone and also not a new Doctor, because she wasn’t the product of a regeneration.
Then we had Smith, Capaldi and Whittaker as 11, 12 and 13.
13 regenerated into 14 (Tennant again). Then the Bi-generation happened, which again wasn’t a regeneration from 14’s POV (same face, same personality), but was a full regeneration in that it produced a new Doctor, number 15.
So, they’re only a new Doctor if they are a product of a full regeneration, who uses the name ‘the Doctor’, isn’t a clone, is a Time Lord, and isn’t a weird spin-off (I’m looking at you, Peter Cushing).
I’m not saying the above makes ‘sense’ outside of the Whoniverse, but I think it’s a reasonably accurate summary of the official position!
That all works, until, as I said in my edit, we now have to count all the doctors before Hartnell that also called themselves The Doctor. Like the Fugitive Doctor.
But the Doctor doesn’t remember them. The Doctor is counting from the first Doctor they remember, which is also our 1st Doctor, and then proceeding from there according to the (made-up!) ‘rules’.
Hartnell will always be the First Doctor, no matter who that character was before that. So we can call Jo Martin the Zeroth Doctor, as some have, and number the rest negatively if we ever learn about them, or we can just refer to them by a descriptor, like the Fugitive Doctor.
It’s also unclear now when exactly the character first took the name “Doctor”, but what is more clear is that the numbering is a convention for the viewers, not really for the characters themselves.