Why were there 13 Doctors prior to #12?

Capaldi is technically the 14th incarnation of The Doctor if you count Tennant staying the same from the Meta Crisis. Matt Smith is supposed to be the 13th and final incarnation of The Doctor, which is why if he had died on Trenzalore that would have been his final resting place...

Now the reason they say all thirteen is because the Timelords on Gallifrey did not know that Tennant's doctor had used up a regeneration to stay the same so they would assume that the last doctor to show up, aka Capaldi, was the regeneration of Matt Smith using a new regeneration cycle.

Because the Meta crisis didn't get involved which means there's only the 1st, 2nd 3rd, 4th 5th, 6th, 7th, 8th, 9th (War), 10th (9th), 11th/12th (10th), 13th (11th), and 14th (12th Doctor, 14th incarnation, 13th face) versions of the Doctor. As someone said, the Meta Crisis didn't have his own TARDIS (they're supposed to take ages to grow anyway).

No 13 Doctors — the 12th Doctor is the 12th Doctor but the 13th regeneration. Moffat said 11th was the 11th Doctor but on his 12th regeneration. Capaldi is part of the new regeneration cycle.

Quote from the Moffat himself:

"He has no more ever called himself the 11th Doctor than he would call himself Matt Smith. The Doctor doesn't know off the top of his head [what number he is]. "If you worry about such things, and I do, then I specifically said John Hurt's Doctor doesn't use the title. [Matt Smith's Doctor] is in his 12th body but he's the 11th Doctor, however there is no such character as the 11th Doctor - he's just The Doctor, that's what he calls himself".

from this article:

http://www.blastr.com/2013-11-25/confused-moffat-addresses-all-those-who-regeneration-limit-questions

Matt Smith was confirmed to be the final incarnation of the Doctor in his original regeneration cycle. The War Doctor counts as one regeneration. The energy used by Tennant in Journey's End was a waste of a regeneration and counts. As Smith's Doctor says in Time of the Doctor. Of course, now things are even more complicated with Jodie Whittaker's recent series finale The Timeless Children where it is revealed the Doctor has infinite regenerations and is not originally from Gallifrey at all. The Doctor was found on another world as a young girl and her DNA was used by a greedy gallifreyan who not only passed on this ability to others on Gallifrey, but erased all memory of the past regenerations prior to William Hartnell. This woman also tortured the Doctor and made her regenerate many times to discover the secret of regeneration. I don't buy this though. Matt is the final incarnation for the reasons I and others have given here.