What will happen when the 13th Doctor dies?

Answer 1

It's actually a she. Not a he. Jodie Whittaker had been selected to play the Doctor after Matt Smith quit. The Doctor switched genders but then regenerated and switched back to male. The actor for the 14th Doctor will be David Tennant (My favorite Incarnation of the Doctor) which also played the Tenth Doctor in the show. He will be returning to celebrate the 60th anniversary of the show in 3 special episodes. The next doctor (15th) will be portrayed by Ncuti Gatwa.

Answer 2

Basically, when the 13th Doctor dies, he will regenerate. They're not going to kill of the main character of the show for the sake of continuity. I think Russel T Davies sneaked in that '503' line into the Sarah Jane Adventures to make it clear that there isn't really a set amount of regenerations he can have.

When they said that he could only have 12 regenerations, they didn't know how successful and how long the show would actually last for.

And of course, when the 503rd Doctor dies... If the show is still successful in about 1000 years time, I'm sure they can rewrite the rules.