How many torpedo hits would it take to sink a ship?

That also depends on a variety of factors. What type of ship was it? Was it in good condition? What was it carrying? Was it fully loaded or nearly empty? What part of the target was hit? Here are some examples:

1. US submarines sank only one Japanese battleship. The USS Sealion (SS-315) sank the battleship IJN Kongo in November of 1944. That required three hits from a salvo of six fired.
2. Carriers normally required multiple hits because of their size. However, the USS Albacore (SS-218) managed to sink the Japanese carrier IJN Taiho with a single torpedo hit. Albacore had help from a fatal mistake by Japanese damage control. More details are below in the Battle of the Philippine Sea.
3. Sam Dealey, captain of the USS Harder (SS-257), was known as the destroyer killer. He was sometimes using “down the throat” shots which meant the destroyer was coming directly at him to attack and he would charge at it. Dealey would fire torpedoes and the destroyer would try to turn away. That actually presented a wider target. Typically, only one torpedo would hit the destroyer. It was a very dangerous method unless it worked.Few other captains used this method.
4. Hitting an ammunition ship could easily result in a huge explosion and a quick sinking.
5. Hitting an escort ship near the stern might detonate their depth charges which would likely sink the escort.
6. Gasoline tankers could explode with one torpedo hit. In that case, the explosion could be a blinding flash and the ship would just be gone when the smoke cleared. However, if it were carrying crude oil, it might not sink unless a fire was started. (Crude doesn’t start burning easily.)
7. An empty tanker could be very difficult to sink unless it was hit in the engineering spaces. Since the tanks were designed to be full of liquid, either fuel or ballast water, a torpedo might only fill them with seawater. Such damage would need to be repaired but might have only slowed the ship a little.
8. The USS Salmon (SS-182) actually sank a ship with a torpedo that didn’t explode. It was an old ship and the dud torpedo punched right through its side. That was enough to flood and sink the target.
9. The Rakuyo Maru, the ship from which the Pampanito rescued the 73 British and Australian soldiers who were POWs, was carrying raw rubber. Although rubber was too dense to float, the fact that it filled that hold and absorbed most of the explosion, left the Rakuyo Maru sinking very slowly. It allowed the soldiers time to climb back aboard and to find a little food, water and materials for rafts before the ship finally sank.