Even though Lucas changed his tune several times as to how many movies he had envisaged, having in magazine interviews et al. even by the 1980s told stories of originally intending 6, 9, or even 12 movies; I think that we can be confident that he *had* always intended more than one movie.
He just didn't expect to get backing for a sequel at the time that he finally got to do the first one.
The original Star Wars was always intended as a pastiche of the Flash Gordon and Buck Rogers serials. Hence the title crawl, and resurrecting the long out of fashion wipe transitions. This was an affectation of being one of many serial episodes.
There was no plan for a sequel. Empire being written almost entirely by Leigh Bracket on a very light synopsis, and originally being a more self contained story. Then Lucas redrafted it to add "I am your father" pivoting the franchise.