Yes, two. But hear me out. If Lucas decides that he doesn't want to continue the story after ROTJ, and he decides that he doesn't want to do a film to show the Jedi Purge and the rest of the Clone Wars (set between 3 and 4), then he could do two different trilogies. And what might they be, you ask? Simple....they would be concept movies.
All the stuff that Lucas didn't use for the OT: Luke was a girl or maybe the Deak Starkiller character; Han was an alien - all the different stuff that already hasn't been used. And he could do the same for the prequels. Anything he didn't use for them, he could put on the screen. Then every fan could see what Star Wars would have been like, had he gone with those ideas.
And he wouldn't have to use real actors. He could do the Final Fantasy type-thing and go all CGI (I know a lot of people have a problem with the way FF was done, but some said Lucas could do that type right, so that's why I'm bringing it up).
So, what does everyone think? I'm not saying that he should definitely do this - it was just something I was thinking. Would anyone support new trilogies like this?