A pair was two individuals, animals, or things used as a group.
In the Talosian menagerie, for them to have descendants of specimens gathered from planets in the vicinity of Talos IV, it was required that they start with a pair, male and female, of each specimen. The Talosians had hoped to add a Human pair to their collection in 2254; however, this attempt failed as Humans were not suitable for captivity. (TOS: "The Cage")
In 3189, Acting Captain Saru ordered that crewmembers work in pairs as they repaired the USS Discovery until internal communications were reestablished. (DIS: "Far From Home")