Here's what I usually do, ymmv:
1 Torpedo:
2 Torpedoes:
3 Torpedoes:
4 Torpedoes:
If I'm shooting more than 4 torpedoes, it's a wide spread to cover my bases in case I miss and it's a high value target. In other words, the spread is greater than the length of the ship. Usually, I plan to hit with 3 in that scenario. Against merchant shipping, when torpedoes are pretty reliable, I usually shoot 2 torpedoes per target. In the early war it's more like 3 for a small target and more for a large target.