Think of the show’s total output as 13 episodes .
Here is the complete, episode-by-episode guide: how many seasons of sherlock are there
Because this special aired between Series 3 and Series 4, many streaming services and casual fans count it as its own “season” (sometimes unofficially called Series 4, making the final run Series 5). However, the creators, Steven Moffat and Mark Gatiss, have always been clear: The Abominable Bride is a special episode, not the start of a new series. Therefore, the canonical count remains (seasons) plus one special. Think of the show’s total output as 13 episodes
The British crime drama Sherlock , created by Steven Moffat and Mark Gatiss, consists of four seasons (referred to as "series" in British parlance). The program aired on BBC One from 2010 to 2017. Unlike standard network television dramas, Sherlock is distinguished by its abbreviated broadcast structure, featuring a limited number of feature-length episodes per season. Therefore, the canonical count remains (seasons) plus one