Maybe one or two top people in the government and secret service of X.
I wouldn't expect too many more than that. It was a secret mission, after all.
But the number of people doesn't matter. It's an all-or-nothing question: Did Atta operate under some outside authority? If so, his actions are part of an overall strategy still being pursued by the larger organization. If not, he just added his personal mark of disorder and death to the world, and the Muslim extremist groups are simply trying to figure out, after the fact, how to build on Atta's actions and take advantage of the resulting situation.
Maybe just the secret service, as opposed to the civilian government or the regular military.
If so, this begs the question, at least for Pakistan, as to who controls the nuclear weapons: the civilian government, the regular military, or the secret service? (Or is it possible that nuclear weapon control there is not unified, that each of these power centers within the government controls some of the nuclear weapons?)