The Office (UK) had two regular series before its Christmas specials.
Each regular series contained six half-hour episodes, giving the main programme a total of 12 episodes. The first series aired in 2001 and established David Brent, Tim Canterbury, Gareth Keenan and the staff of Wernham Hogg. The second series followed in 2002 and continued the documentary crew’s observation of life at the Slough branch.
After series two, Ricky Gervais and Stephen Merchant decided not to make a conventional third series. Instead, they wrote two longer episodes that became the Christmas specials. These were broadcast on BBC One on 26 and 27 December 2003 and served as the programme’s finale. The specials are sometimes counted as a third season by streaming services or international listings, but that is a classification difference, not evidence that a third regular series existed.
The phrase “regular series” is therefore important. The canonical British run consists of two six-episode series, followed by a two-part Christmas special. The specials also expanded the story by revisiting the characters after the events of the second series.