Only 18 people survived the attack on the World Trade Center on September 11, 2001. There were more than 2,000 people present above the point of impact, and about 1,100 people are believed to have survived the initial crash, but very few walked out alive after the second crash.
The workers in the South Tower were asked to return to their desks, since the building was believed to be secure. However, most of them wanted to evacuate the building anyway. That is when the second jet crashed into the tower, causing explosions on the 78th floor. Although there were hundreds of people on that floor, only two survived.