Come on, it only takes one person with a knapsack. This is like those claims that it would have required tractor beams to levitate the stones of the Great Pyramid into place -- when there are murals from ancient Egypt showing entire statues of greater mass being pulled around on sleds by rope teams.
They lost 700 employees working on four floors. Even accounting for their being on top floors, you take 220 storeys and do the math, you get a figure of 42,000.
Actually, 38,500. But they've only found 400 bodies at Ground Zero. Doesn't that tell you that most people evacuated?
No, it doesn't . . . not with any degree of certainty.