The Office (UK) produced 14 episodes in total when the two Christmas specials are included. The programme consists of two regular series with six episodes each, followed by a two-part Christmas special.
The first series aired in 2001 and contains six half-hour episodes. The second series aired in 2002 and also contains six episodes. After those series ended, Ricky Gervais and Stephen Merchant wrote two longer Christmas episodes, broadcast on 26 and 27 December 2003.
This is why totals can sometimes appear inconsistent. Counting only the regular series gives 12 episodes, while counting the Christmas instalments separately brings the total to 14. The Christmas episodes are not a single episode: they are two distinct parts, each with its own broadcast date and episode number.
The 2013 charity short The Return of Brent, also known as The Office Revisited, is generally treated separately from the original run. It was made a decade after the Christmas specials and does not change the standard answer for the original programme’s episode count. The complete DVD collection likewise lists 14 episodes.