>>7328254 (lb)
Medfag here...
It depends upon the injuries. A single person with a large vessel injury or gunshot to the liver or spleen could require 20+ units during surgery even with interoperative salvage.
Blood that is donated is not taken to the hospital. It is tested over the next day or 2, processed into packed cells, platelets, FFP, Cryo, etc and then sent to other blood centers around the country who are short.