Answer: He played eight different characters.
In this movie set in Victorian England, Alec Guinness plays every member of the aristocratic D'Ascoyne family, both male and female, young and old!
The villain, Louis Mazzini (Dennis Price), is from the wrong branch of the family and would need eight D'Ascoynes to die before he's next in line to become the Duke, so he resolves to bring about the premature demise of all who stand in his way. During the course of the story we see him cleverly (and humorously) despatch six D'Ascoynes, one after the other, each victim played by Guinness (sometimes in drag). We see the last two D'Ascoynes perish without Mazzini's assistance, allowing the villain to inherit the title of Duke of Chalfont. Later, there's poetic justice when Mazzini ends up on trial for a murder that he didn't commit!
Alec Guinness (1914-2000) had a long career in the movie business, playing a wide variety of roles. He was a versatile performer who moved easily between Shakespeare, drama, comedy, action films and sci-fi. Younger people may only remember Guinness from the original "Star Wars" trilogy, in which he played Obi-Wan Kenobi.