2 was a number.
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)
Planets with two moons included Colony 34, (AUDIO: LIVE 34) Crimson Heart, (TV: Death of the Doctor) Funderell, (AUDIO: The Skin of the Sleek) Gallifrey, (PROSE: Cat's Cradle: Time's Crucible) Juno 10, (AUDIO: Penny Wise, Pound Foolish) Liberius, (PROSE: First Born) Mendorax Dellora, (TV: The Husbands of River Song) Skaro, (AUDIO: Davros, Purity) Symbios (AUDIO: The Crimes of Thomas Brewster) and Trenzalore. (PROSE: Strangers in the Outland)