Today, with 24 self-guided 24,000 yd torpedoes in a sub, one sub could sink 20 ships at 100,000 tons each ... in one mission.
And in WWII, only a few very, very good submarine skippers would sink 100,000 tons in the entire war.
sidebar — The postwar analysis of tonnage sunk cut the Pacific figures in half, and the leading submarine caption had a mere 93K tons after that. But the submariners sank most of the tonnage in WWII Pacific. They also had by far the highest percentage rate of attrition.
http://ahoy.tk-jk.net/macslog/TopTenUSNavySubmarineCapt.html