Each crew has four men, giving the U.S. 3,808 M1 tank crewmen. The army is hustling to retrain all these tank crews for conventional combat. During the last decade many tank companies were used as infantry or to operate MRAP vehicles. The army is using lots of simulators to retrain the tank crews and this cuts costs a lot. These simulators have become more common since the 1990s and have proved to be very effective in quickly and cheaply teaching useful skills to tank crews. After lots of simulator time, the crews perform very effectively when they take the tanks out and do all the moving and firing under realistic (or even combat) conditions.
All those additional M1 tanks are there if there is a major war. With all the simulators it is easier and quicker to train more crews than it is to build more M1 tanks.