Technically 3. What happened was after the first two collapsed, a no more than 4 story building fell due to the rubble and fire is the two. Now there was also propane storage in the basement of this bulging (known as WTC3* I believe) that also helped with the collapse. Also part of the pentagon was attacked wrecking one whole side. The third plane was brought down into a field in Pensilvania due to the help of passangers that stopped the hijacking.
*World Trade Center 3
*World Trade Center 3