Lucas made the claim shortly after Empire Strikes Back came out. He said it once and it spread like wildfire. He never mentioned it again in all those years.
He must have figured that he'd have the prequels all done by 1990 and that he'd have plenty of time to get a good idea for a follow up. He never got beyond "Shift everything twenty years into the future, Luke is now a Jedi knight and they are rebuilding the republic" Knowing George and his talent for plots, it's clear that it would be just as messy as the prequels.
Then things broke down for Lucas, Marcia left him, the Kutzes were out of the picture and George sat on his laurels and concentrated on the prequels. It took him longer than he originally expected. Also the original trilogy bothered him. It was the work of other people who had disappointed him bitterly, he didn't like the OT and he started to change it so that it would be more "him" and airbrush out Marcia Lucas' work. By the time Return of the Jedi rolled out he was backpedalling on his his claims for a sequel trilogy and wasn't even certain he wanted to invest in the prequels.
By the Time George felt comfortable tackling the Prequels he had long abandoned the idea of the sequels. The prequels were his baby and his baby only, that's why current emphasis in Star Wars is 75% prequels, and 25% original trilogy. The sequels would connect to the films that made him rich and successful, but also gave him the most unhappy period of his life.
Lucas had some ideas, but nothing really good and useful for another trilogy. By the time the last prequel came out Lucas told himself and the world that the saga had come full circle.
Meanwhile the fans believed everything Lucas told them and over time it became dogma that Lucas had promised them 9 films and there would be nine films, not eight, nine being the number you are looking for. Ten being too much of a good thing and eleven would be a serious disturbance to the Force. Nine therefore it is, shall be and forever be, May the Force be with you.
Then Lucas decided it was time to get away from Star Wars and sold it to Disney. He probably put in a contract clause that he keeps final say about the original movies, but that the rest would be pretty much fair game. So Disney remembered the 9 film promise and used this as the sales pitch of the decade …
