Not necessarily.
10 to 15 people would be enough to plan and execute it.
I don't hold much truth to US government officials executing the attacks. What seems more probable to me is that intelligence services were aware of the impending attack (they were warned by other countries.. and well, there's the stories of people being told not to go to work.. if these people knew, there's a very reasonable chance the US intelligence also knew). So yeah, you don't need to make 911 happen, you just need to enable it to happen (provided you've got a good agenda to make the sacrifice & risks worthwhile).
Such an operation wouldn't require too many people. Some 10 conspirators at the top
wanting it to happen, and some 10 more to execute it (that is, keep the fighters away, mislead the national defence, formulate a story to be brought to the media right after the attacks, etc.). And if you want to, add a few more for additional purposes (destroying WTC7, pentagon??).
Most people involved can be used as pawns. They can be made to do things without being aware of their part in the scheme.
Anyone who watched the movie Valkyrie about the coup of Stauffenberg & co against Hitler knows this.
The list of 'conspirators' back then was very very long:
http://en.wikipedia.org/wiki/List_of_members_of_the_20_July_plot And yet, Hitler was caught by complete surprise.
Nevertheless, at the same time, how many people were really needed to execute the operation?
One man to plan it. (Stauffenberg).
One man to get the means (the bombs).
One man to plan the bomb (Stauffenberg again) and one man to assist him.
One radio operator to keep messages from going out of the Wolfsschanze.
A small selection of men ready to take government in their hands.
I don't think more than 25 people played any significant role in the event.
Same thing could have been on 9/11. You really don't need thousands of people.