When a Time Lord is mortally wounded or hurt, they are able to use a process called regeneration. Regeneration is a process in which all the cells in the body are replaced with new ones. The result is a completely new appearance. Since the brain cells are revived, a change in personality usually occurs. Even though the Doctor has changed several times, he still retains the same memories of his previous selves and remains a force for good, even though his priorities sometimes change.According to the 1976 storyline "The Deadly Assassin" (and since reaffirmed several times in the 1963-89 series and 1996 TV movie), a Time Lord can only regenerate 12 times, for a total of 13 incarnations. However, the 2013 Christmas Special "The Time of the Doctor" changed this so that the Doctor now has 12 more regenerations.
In "The Day of the Doctor," from which points in both the classic & new series did the other twelve Doctors arrive?
According to the Doctor Who Wiki...First Doctor: Between "The Daleks' Master Plan" (& "The Five Doctors" & "The Massacre of St Bartholomew's Eve".Second Doctor: Between "The War Games" ("The Two Doctors") & ("The Five Doctors") "Spearhead from Space" because of his work for the CIA (Celestial Intervention Agency) which delayed his regeneration.Third Doctor: UNDATEDFourth Doctor: UNDATEDFifth Doctor: UNDATEDSixth Doctor: UNDATEDSeventh Doctor: UNDATEDEighth Doctor: If all you count is TV, then between Doctor Who & Doctor Who: The Night of the Doctor. However, since the audio adventures are now considered canon, then between "Benny's Story" & "Terror Firma", although, if you count all expanded universe items, then between the short story "For the Man Who Has Everything" & the novel "Alien Bodies".War Doctor: Technically, after Doctor Who: The Night of the Doctor. If you count the expanded universe, though, then after the novel "Engines of War".Ninth Doctor: UNDATEDTenth Doctor: Between "The Waters of Mars" & "The End of Time Part 1"Twelfth Doctor: UNDATED
How did The Eighth Doctor regenerate?
In the middle of the Time War, The Doctor died while attempting to save a pilot about to crash into the planet Karn. Revived by The Sisterhood and only having 4 minutes to live, he had the choice of an elixir to force the regeneration into anything he wanted. Having been convinced that he was needed to end the Time War, he chose to be The Doctor no more and become a warrior.Regenerating into the War Doctor, he set out to end the war once and for all.