Before the The Timeless Child was introduced in season 12, there was plenty of speculation on how many lives or regeneration’s The Doctor would have. Especially after The Time Of The Doctor when the Time Lords granted the 11th Doctor more regeneration cycles. In Hell Bent, even the Lord President of the Time Lords said he didn’t know how many he was given.
I myself suggested 30 because that is how many desktops The T.A.R.D.I.S. said she had in The Doctors Wife.
In Kill the Moon, The Doctor told Lundvik it would be time consuming to kill him because he would keep on regenerating and “I’m not entirely sure I wont keep on regenerating forever.”
Thus foreshadowing the idea that The Doctor had infinite lives.
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