For me, it depends on targets, how many torps to sink following;
Medium cargo - 2 torps typically
Empire type frieghter - 2 torps typically
Large Merchant - 1 torp typically, rarely takes 2
C3 - 2 torps typically
Large tanker - 1 torp if it load with fuel, if not, it may take 2 if it not sinking
Modern tanker - same as large tanker
Whale factory ship - same as large tanker
Troop ship - 2 torps typically
Large troop ship - 1 torp if its stationary, fire another one if it doesn't sink, but if its sailing on sea, then 2 would be better
Ammo ship - 1 torp if hit right and it blows up, so 2 would probably be safer.
destroyers or escorts - 1 torp always
battleships - depends if it stationary or sailing and its hull protections. Few days ago, there were 3 battleships at high speed NE of Ireland, Hood, Nelson, & Revenge. I fired only 3 torps, one for each battleships, they all hit their bows near fronts. Both Revenge and Nelson's bows went into water and sank, but Hood slowed down to 7 knots and getting away while their escorts were after me. So I should have fire 2 at Hood in first place.

After the escorts lost me and went away, I chased that Hood, the escorts went off somewhere else. I was around 16,000m away before Hood disappeared at Firth of Clyde. Oh well, there will be next time anyway.
For small ships like small/coastal merchants, I use deck gun on them or I don't bother with them if they have guns or there are planes & warships nearby. I prefer 1 torp per 4,500t minimum. I know in real life Kaluens used torps on any ships regardless of their tonnages. I would do the same if in real life cuz no TC. I probably would be tempted to use a spread of 4 torps at a 2,000t merchant to deplete torps sooner so I can return back to port to get breaks from stress, dangers at sea, the reeks of all men in a tight ship, and to enjoy fresh food and women. Or I could go flank speed all way to patrol zone using up fuel giving me excuse to return back to port. They probably would demote my rank then. Of course this game is fun, but not for real life.