I ran a lot of tests on sinking when I worked on my sinking mod.
All IJN BBs, except the Yamato class, can be sunk by 2-4 torpedoes. It is all a question of where you hit. 2 well placed torpedoes is usually enough. Remember that the damage caused by a torpedo is variable and all ships are broken down internally into various watertight compartments, which is why sometimes 2 is enough, sometimes you need 3-4.
The Yamato/Musashi are in a different class. I have never been able to sink one with less than 6 torpedoes, sometimes it can take as many as 10.
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