Animal Trainer Julie Scardina, followed by pro football Hall of Famer and analyst Terry Bradshaw.
Actually the above answer is not accurate, [going by imdb's credits] the person who made the most appearances on Leno's Tonight Show was Robert Covarrubias. He appeared as various characters in sketches, followed by Adam Smith & Shannon Ratigan, then animal expert Julie Scardina, followed by Nina Bergman, then Ross Matthews and then Terry Bradshaw.
Although if the question is strictly referring to celebrity appearances & not people who appeared on camera who worked for TTS, then Julie has the most appearances.
He was an American television host and comedian, known as host of The Tonight Show Starring Johnny Carson for 30 years (1962-1992)
You click on the price and download it.
In November 1980, Joe Newton appeared on The Tonight Show starring Johnny Carson.
Johnny Carson would enter the stage through the curtains.
The Johnny Carson Show aired from 1955 to 1956.
The duration of The Tonight Show Starring Johnny Carson is 1.75 hours.
The Tonight Show Starring Johnny Carson was created on 1962-10-01.
The Tonight Show Starring Johnny Carson - 1962 was released on: USA: 1 October 1962
The Johnny Carson Show - 1955 1-10 was released on: USA: 1 September 1955
The Tonight Show Starring Johnny Carson - 1962 1-10 was released on: USA: 12 October 1962
The Tonight Show Starring Johnny Carson - 1962 1-11 was released on: USA: 15 October 1962
The Tonight Show Starring Johnny Carson - 1962 1-12 was released on: USA: 16 October 1962
The Tonight Show Starring Johnny Carson - 1962 1-13 was released on: USA: 17 October 1962
The Tonight Show Starring Johnny Carson - 1962 1-14 was released on: USA: 18 October 1962
He was an American television host and comedian, known as host of The Tonight Show Starring Johnny Carson for 30 years (1962-1992)
The Tonight Show Starring Johnny Carson - 1962 1985-01-02 was released on: USA: 2 January 1985
The Tonight Show Starring Johnny Carson - 1962 1985-01-08 was released on: USA: 8 January 1985