Two per target makes sense. You can straddle the ship's midline and thereby hedge your bet on its speed (if you're slow you turn it into a midship + stern hit, if you're fast you turn it into a midship + bow hit). Two hits almost always kills a ship. If it's bigger (like a BB, or an aircraft carrier) I'll give her three. If I'm really unsure of the speed of a BB (which sometimes happens because they go so fast) I'll give her four, spaced evenly along her. In the IX, two per target is great because it means 3 ships, almost certainly destroyed, per convoy raid. In convoys, it's not worth shooting one torp at a target because if it takes her days to sink, you have to hang around to make sure, and thus lose distance on the rest of the convoy. You want to inflict sudden destructive hits on all targets, and then start the overhauling maneuver. I don't really know what they did in real life
, but I sort of imagine that having played the game for long enough on high realism, you'll basically end up behaving like a real Kaleun.