The liner notes for The Sergio Leone Collection says there are nine movies in Serio Leone's "official canon". It seems like it should be right.
Ok, let's start with The Colossus of Rhodes. Then there's the "Dollars Trilogy", A Fistful of Dollars, For a Few Dollars More and The Good, the Bad and the Ugly.
We're at four now, and probably not starting any fights.
Now we come to the "Once Upon a Time Trilogy", Once Upon a Time in the West, Duck, You Sucker and Once Upon a Time in America.
(The original title of Duck, You Sucker was "Once Upon a Time... The Revolution", for those needing an explanation there.)
Now, we're at seven.
And we're at the end of what I understand as the "official" canon.
So, we come to the other possibilities...
The Last Days of Pompeii by Mario Bonnard, which Leone apparently began as assistant director and completed as director, after Bonnard fell ill.
My Name is Nobody by Tonino Valerii. This movie was conceived and produced by Leone, and, by most accounts, he did direct the opening sequence.
Now, there we have nine and this is probably what that "official" nine refers to.
But it leaves out A Genius, Two Partners and a Dupe by Damiano Damiani, which apparently also has some uncredited input from Leone.
Not to mention Hanno rubato un tram and Il cambio della guardia, both of which are listed on IMDb.
The trouble lies in the "official". If they said "nine movies to his credit", I'd have figured out which nine it most likely meant and moved on. Official canon leads to questions of whose official canon and where it draws what lines.