On September 11, 2001, a total of three buildings in the World Trade Center complex collapsed due to the terrorist attacks. The most notable of these was the North Tower (WTC 1), which was struck by American Airlines Flight 11 at 8:46 AM and collapsed at 10:28 AM. The South Tower (WTC 2) was hit by United Airlines Flight 175 at 9:03 AM and collapsed at 9:59 AM.
In addition to these two towers, World Trade Center 7 (WTC 7), a 47-story building located adjacent to the Twin Towers, also collapsed later in the day at approximately 5:20 PM. This building was not struck by an aircraft but suffered significant damage from debris and sustained fires that burned for several hours.
In summary, the total number of buildings that collapsed on that tragic day was three: the North Tower, the South Tower, and WTC 7.