Number 28 was the title of one of The Village's supervisors. The character appears in the first episode ("Arrival") and subsequently appeared in the episodes "The Chimes of Big Ben", "Free for All", "The General", "Hammer Into Anvil", "It's Your Funeral", "A Change of Mind", "Once upon a Time", and "Fall Out". Credited as Supervisor, the character was portrayed by actor Peter Swanwick.
The bald, bespectacled control room supervisor is frequently seen, and is played by Peter Swanwick. He would appear to be the continuing right-hand man (in terms of surveillance and security) for whomever is Number 2. The Supervisor was actually fired by one particular Number 2 (played by Patrick Cargill), but was then apparently quickly reinstated after that particular Number 2 was replaced in disgrace.
Though the Supervisor has a number on his badge, he is never addressed by number and the number is never referred to in conversation. His number is clearly seen to be 28 in "The Chimes of Big Ben", "Hammer Into Anvil", "It's Your Funeral" and "Once upon a Time". However, in "Fall Out", which takes place directly after "Once upon a Time", an extremely brief glimpse of his badge can be seen; in that episode, it reads 27. This can be most likely attributed to the props department as an unintentional continuity error.
Note that a few episodes have other people performing the function of the control room supervisor. These supervisors are clearly not meant to be the same character as Swanwick's supervisor -- in fact, one new supervisor explicitly takes over for Swanwick's supervisor after he is fired. These other supervisors might be simply working a different shift from Swanwick's supervisor, as the control room is operational 24 hours a day. It's also possible that Village authorities periodically rotate different people through the position of Supervisor, as they seem to do with the position of Number 2. As with all the Village supervisors, he is identified as such by his moss green turtle neck jersey.