If it's ever shown or revealed that the Master had an incarnation after the Delgado one that became the burned one, I have no problem accepting Delgado was the Twelfth, or even earlier. I read that a novel established he was in his final incarnation due to some encounter with the Second Doctor, or something? I haven't read it yet, and may never, because I find Doctor Who
novels difficult to read.
Edit: Also, I don't really like referring to the incarnations of the Doctor as the actors, so I try to apply the same terminology for the Master and Romana. I hate equating characters and actors other than for the occasional in-joke reference. I especially hate it when a character is called the actor as if that's funny.