That comes to 500,000 divided by 220 floors (110 stories each tower).
Thats 2300 people per floor? Nope...dont think so, not even when you count the ancillary buildings, such as the 47 story Tower 7 etc.
On the other hand, if you think about 24 hour a day operations, you get three shifts of eight hours, and that would alter your figure to about 800 per floor. Also: there may have been lots of part-time workers who may or may not have been there this morning.