BBC’s Sherlock has twelve standard episodes across its four regular series.
Each series contains three feature-length episodes, so four series multiplied by three gives twelve. The format made the programme unusual for a continuing drama: instead of producing many short weekly episodes, the BBC presented long, self-contained mysteries with continuing character arcs.
The total is often reported as thirteen because “The Abominable Bride” was broadcast as a standalone special in January 2016. That story belongs to the Sherlock television universe but is not one of the twelve episodes assigned to series one through four.
The standard episode count therefore excludes the special. Including every broadcast story, the programme has thirteen installments; counting only regular series episodes, the precise answer is twelve.