That is an awfully good question that has never really been answered, to my knowledge. I've seen some persuasive evidence there was at least one more.
I also think the French documentary is essential viewing, even though it makes me sick to watch those brave firefighters disappear into the stairwells, knowing many will not be seen again.
I also agree there were some miracles that day that limited the damage done, although that seems to have been interpreted by many to mean we can go back to the head in the sand approach of pretending they don't really want to kill us.
I think the succesful manner in which we have been taking it to the terrorists has gone a long way in not having any more serious attacks on the U.S. Some shootings (Utah mall) and some vehicle vs. pedestrian attacks may have been motivated by Islamic hate - but not a coordinated or serious effort.
But - the lack of terrorist attacks is what I fear will make our new leaders complacent.