That was really about the fleet boats. For US S-class the sources vary a bit. It varies from 35 (S-18) to 45 (S-42). However, an account of the sinking of the Squalus (SS-192), states 59 hands on board. But the Squalus sank on a test dive, so maybe less crew would actually be carried into battle. IMHO setting sail with 50 hands on board shouldn't be too far from reality.
*Nevermind on the Squalus; she was Sargo-class. Still, 42 to 45 men, maybe a bit more should be alright I think.