13 (number)

13 was a prime number, (AUDIO: The Haunting) considered unlucky by the superstitious, (COMIC: Triskaidekaphobia) due to "mystic properties". (AUDIO: The Thirteenth Stone) Some people were beset by triskaidekaphobia, according to the Twelfth Doctor an irrational fear of the number 13. (COMIC: A Confusion of Angels)

In particular, the Triskaidekaphobes, a group of thirteen clones, feared the number above all else. Living, as they did, on the 13th moon of the 13th planet in the 13th Galaxy, they considered themselves the unluckiest people in the galaxy.

When the Tenth Doctor visited their moon with Rose Tyler, on the 13th day of the 13th year in the 13th century, a Friday, they discovered that the gambler Bob Kreesus had been extracting all good fortune from the Triskaidekaphobes with a machine he had won, in order to bring himself good luck. (COMIC: Triskaidekaphobia)

According to Rose Tyler, Friday the 13th was considered especially unlucky. (COMIC: Triskaidekaphobia)

Regeneration
Time Lords generally had up to thirteen lives, as they were afforded twelve regenerations. (TV: The Deadly Assassin, The Five Doctors, AUDIO: Trial of the Valeyard) Some individuals, like the Doctor and the Master, were given additional regeneration cycles, to allow them to go on past their natural ends. (TV: The Time of the Doctor, The Sound of Drums) The High Council of Gallifrey were empowered to offer such individuals new life cycles. (TV: The Five Doctors)

After being gifted a new cycle by the Time Lords on Trenzalore, the Doctor regenerated for the thirteenth time, into the Twelfth Doctor. (TV: The Time of the Doctor)

Behind the scenes
Though unremarked in PROSE: Daisy Chain, 13 is also the seventh number in the Fibonacci sequence.

Triskaidekaphobia, centred on unlucky number 13 at galactic coordinates 13:13:13:13:13:13, was published in the 13th issue of Doctor Who Adventures.

The dialogue in A Confusion of Angels which elaborates on triskaidekaphobia was released in the final issue of Titan's ongoing Twelfth Doctor series — specifically Year 3 #13 — before the launch of the Thirteenth Doctor's own range.

The trailer for Meet the Thirteenth Doctor, when Jodie Whittaker would be announced as the Thirteenth Doctor, placed the number 13 in a variety of places around the world.