Prior to the engagement, Custer split the 7th into 3 combat commands for tactical reasons. Custer took 7 companies off to the northwest to attack the encampment. Those 7 companies under his direct command were wiped out. The remaining companies under Reno and Benteen would retrench on heights east of the encampment, and survive.
Thus Custer's command suffered 100% casualties, even though a slim majority of the regiment survived.