I have this vague recollection of something I read, back in 1977 or 1978, that had him projecting a dozen movies. Then, around 1980, I distinctly recall reading that he was talking 9 films. I don't believe he started swearing up and down that he'd only planned on 6 until well after RotJ was released, maybe not until he started working on TPM.
References to 12 movies show up in "Bantha Tracks", the official Star Wars Fan Club newsletter. In #3 (July 1978), there's a statement that says "The sequel [TESB] will be based directly on the second of twelve stories George Lucas wrote in the Adventures of Luke Skywalker
". Then in #8 (Spring 1980), in an interview with Lucas, the interviewer states "At one point there were going to be twelve Star Wars
films", to which Lucas replied "I cut that number down to nine because the other three were tangential to the saga." These are the obvious references to 12 or 9 films that I am aware of, but there may be others out there in that time frame.