2 (number)

2 was a number.

7 was not divisible by 2. (PROSE: And Then Again)

The Ux were a duo-species, meaning there were always only two of them. (TV: The Battle of Ranskoor Av Kolos)

The Boneless came from a universe with only two dimensions. (TV: Flatline)

The Twelfth Doctor once told Bill that "eleven plus two" was an anagram of "twelve plus one", to which Bill responded that both of these were equal to 13. (COMIC: Harvest of the Daleks)

Humanoids had two arms, and were bipedal, meaning they had two legs, and binocular, meaning they had two eyes. (TV: Kinda) They also had two ears. (TV: Rose) The resulting symmetry played a large part in attractiveness. (TV: World Enough and Time)

Time Lords had a binary vascular system, meaning they also had two hearts. (TV: Spearhead from Space, et al.) On discovering the Tenth Doctor had two hearts, Jackie Tyler wondered if there was anything else he had two of. (TV: The Christmas Invasion)

Behind the scenes
Though unremarked in PROSE: Daisy Chain, 2 is also the third number in the Fibonacci sequence. Also, unremarked in AUDIO: The Haunting is that 2 is a prime number.