Depending on his plans at the time and when you asked him, George Lucas has variously said there would be one film, three films, six films, nine films or 12 films.
1980
It was widely reported in 1980 that there would be nine films: a trilogy of trilogies.
Al Walentis wrote in the May 25, 1980 Reading Eagle:
Lucas originally envisioned "Star Wars" as a single feature, but his 200-page screenplay proved too unwieldy. He then began tinkering with his story line, cutting it apart, sorting our all the various subplots. The script finally was pieced together as three distinct trilogies.
"There are essentially nine films," Lucas said. "The first trilogy is about the young Ben Kenobi and the early life of Luke Skywalker's father when Luke was a young boy. The first trilogy takes place some 20 years before the second. About a year elapses between each story of the first trilogy. The whole adventure - encompassing the three trilogies - spans about 40 years."
Irvin Kershner, director of The Empire Strikes Back (1980), was also on-script with the nine-film plan. Tom Buckley of the N.Y. Times wrote on May 25, 1980's The Spokesman-Review:
"I told George I didn't want to do a sequel," Kershner said."He said, 'I don't blame you. Neither would I, but this isn't. It's the second act of the second trilogy of nine films I plan to make in this theme. I want it to be better than mine.' ..."
On the following page of the same paper, Aljean Harmetz of the New York Times wrote:
The "Star Wars" George Lucas has created in his mind will take nine movies to tell. "Star Wars" is actually "Star Wars, Episode IV: A New Hope," the first movie of the second trilogy. "The Empire Strikes Back" is "Star Wars, Episode V," while "The Return of the Jedi' is episode VI. The first trilogy deals with the young Darth Vader and the young Ben Kenobi. At the end of the first trilogy, Luke Skywalker is four years old. Only the robots - R2D2 and C-3PO - will be characters in all the movies.
He chose to start in the middle because the first trilogy is, he says, "more plot-oriented, more soap-operaish." He adds that the "central core problem" of "Star Wars" hasn't even been stated yet. Although he originally saw Star Wars as six movies, his "dream" was only for "Star Wars" to do well enough so that he could finish the three movies in the second trilogy. "If people has laughed 'Star Wars' off the screen, I'd have been less surprised than I was at what did happen," he says. "Until the say it opened, I felt it would do $16 million and, if I pushed hard, I could make 'Empire.'"
An interview with Harrison Ford in Lakeland Ledger of July 4, 1980:
Like Mark Hamill (Luke Skywalker) and Carrie Fisher (Princess Leia), Ford has already signed up for the third "Star Wars" film, which is tentatively entitled "The Revenge of the Jedi." This will conclude the middle trilogy of the nine-part series, and Ford does not know whether it signals the end of Han Solo. It is up to George Lucas, the creator of the saga.
"He has an idea of doing one (involving what happened to Solo) about 15 years from now, when I'll be 53. That's something I'd like to do."
The Phoenix of June 21, 1980 also mentions the three trilogies spanning 40 years, and:
Thereafter, Lucas will go back to the first trilogy, starting with a story so far back that it does not include Darth Vader.
If interest sustains, Star Wars could be in production well into the 1990s. David Prowse figures he'll get killed off (by Luke Skywalker?) in the seventh or eighth story.
1977 & 1978
And it seems at least one trilogy was planned from the start. One week after Star Wars came out, The Leader-Post of June 3, 1977 says:
[Mark] Hamill said he believes Lucas plans a Star Wars trilogy because all the actors are under contract for two more films.
Reporting on the film's record-breaking success, an AP story in May 26, 1978's Schenectady Gazette says:
Lucas had originally conceived of "Star Wars" as a trilogy. Work on the first sequel is well under way...