The September 11, 2001 terrorist attacks killed 2,977 people, not counting the 19 hijackers. That total covers people killed at the World Trade Center in New York, at the Pentagon in Arlington, Virginia, and aboard the four hijacked planes, including United Airlines Flight 93 near Shanksville, Pennsylvania.
The National September 11 Memorial also lists six people killed in the February 26, 1993 World Trade Center bombing, for 2,983 names inscribed on the Memorial. The 2,977 figure is the standard count for deaths resulting from the September 11 attacks themselves.
Source: National September 11 Memorial & Museum — The Memorial