There is no clear agreement on this. Some people count Season 23 as one story("The Trial of a Time Lord"). Others count it as 4 different stories("The Mysterious Planet", "Mindwarp", "Terror of the Vervoids", and "The Ultimate Foe"). But the Production Team originally counted it as three stories, because what many fans today call two different stories("Terror of the Vervoids" and The Ultimate Fore) was counted as a single story called "The Ultimate Foe", as the Production Number and Notes show.
What many people today call a standalone story("Mission to the Unknown") was actually Episode 5 of the Story "Galaxy 4". However, many people today count "Mission to the Unknown" as part of the later Story "The Daleks' Master Plan".
The UK and US DVD releases of the Original Series use two different numbering systems for Original Series Stories.