I agree with Shinigami. FFC saying "There should have been only one." doesn't really say much as to what he meant. One what? One Godfather movie? One sequel?
I wish Coppola would've said more as to what he meant, but instead we get TMZ getting an entire article out of six words that all of a sudden has caused an uproar among movie fans.
I find it a little hard to believe if Coppola actually meant one Godfather movie, considering the first movie only adapts half the book (the second movie covers the other half). I'm not completely believing TMZ on this one.