Article: Fifty Years of The Master
There are four actors playing The Master in the original series, but I only remember Delgado and Ainley. Pictures of all eight.
https://www.doctorwhonews.net/2021/01/fifty_years_of_the_master.html
Created by producer Barry Letts and script editor Terrance Dicks, the character was devised as a foil to the Third Doctor as played by Jon Pertwee. Given an academic name. The Master, to counterpoint the name of The Doctor, the character was to be the Moriarty to Pertwee’s Holmes, a character very much the equal to the Doctor in terms of intelligence and background.
The character outline was sketched out in an internal BBC document sent to the writers of Season 8 of the series, a document which also introduced the characters of Jo Grant and Captain Mike Yates, played by Richard Franklin and Katy Manning.
Only one actor was considered for the role of the Master, Roger Delgado, an actor known for playing sinister villains and a personal friend of Jon Pertwee.