A RL reality check:
For the entire war, USN SS attacks on Jap AOs and XAOs of ~10k tons:
0% of the ~10k AOs hit by 1 fish sank.
63.6% of those hit by 2 fish sank.
66.7% of those hit by 3 fish sank.
100% of those hit by 4 or more fish sank.
Note that a "hit" means a detonation on the target

Early war skippers unfortunately had many, um, torpedo impacts, or torpedoes crossing the target but not detonating.