I'm not a naval historian, but I am a former Royal Navy officer who served in two submarines (and has read a bit about the Battle of the Atlantic), so perhaps I can correct some of the apparent misconceptions in the question, and give a bit of context, while waiting for a better answer to come along. I hope the Moderators will allow that.

It's a misunderstanding to think of submarines as fighting 'for a very short amount of time', as if they were battleships engaged in a sustained gun exchange. That's not how submarine warfare works.

The primary target of German submarines in WW2 was allied merchant convoys. The Battle of the Atlantic was a campaign of strategic logistics: without materiel from the farms and factories of North America, the allies would not have been able to wage war in the West.

Occasional specialist convoys would have a capital ship (battleship, battlecruiser, or aircraft carrier) attached, as was the case Operation Pedestal convoy to Malta in the Mediterranean campaign. Normally, however, convoys in the Atlantic were escorted primarily by smaller ships equipped (sometimes hastily) for anti-submarine warfare. These sloops, corvettes, frigates and destroyers would typically have ASDIC (a simple active sonar for finding submerged submarines), HF/DF ('high frequency direction finding', which could detect the radio emissions of submarines), a primitive radar (from about the middle of the war onward), depth-charge racks, and one or more guns firing 4" to 4.7" shells. That's enough for them to do serious damage to a submarine, if they can find it.

The primary attribute of a submarine is its ability to evade detection. As long as it is hidden, it is extremely powerful. Its power lies in the fact that, undetected, it could be anywhere. Just the threat that submarines might be operating in an area hinder the ability of ships to pass freely through that area. Once its position is revealed, however, a submarine (of WW2 vintage) is relatively vulnerable. Early in the war the Admiralty over-estimated the effectiveness of ASDIC, although later tactics - including having escorts working in pairs so that one could maintain ASDIC contact while the other pressed him the attack - made it moderately effective. Another innovation in the middle of the war was anti-submarine mortars, which fired a depth-charge a short way ahead of the vessel, so that ASDIC contact would not be lost. A found submarine could soon become a dead submarine.

It's also worth noting that WW2 submarines were slow underwater - an escort destroyer could literally run rings around them. They had to surface generally every 24 hours in order to recharge their batteries and refresh their air. This added to their vulnerability if detected.

All of this is a prelude to the main point, which leads to the answer to your question: one of the quickest ways for a submarine to reveal its position is to fire a torpedo. Torpedoes are really noisy. When they are fired, they are pushed out with a big surge of compressed air, which is also really noisy (remember, the primary 'sense' for underwater warfare is hearing; a passive sonar / hydrophone can pick it up, depending on the water conditions, from miles away). The torpedo runs close to the surface, leaving a visible trail that can be spotted (if conditions are favourable) by an alert lookout. Even if all those clues are missed - which they might be, on a dark, cloudy, moonless, squally night in the North Atlantic - then a great big explosion blowing the bows off a precious Liberty Ship carrying Canadian wheat and Detroit-made machine parts to England is a very big give away that a submarine is nearby. Once the attack has been launched - even if it successful - the submarine has entered a game of hide and seek where its one objective is to escape with its life.

The priority, then, is for the submarine to do as much damage as possible in its initial attack. Torpedoes would usually be fired in salvoes - three or four at a time - in a spread; this not only hedges bets in terms of calculating the 'firing solution' (range, speed, angle-at-target-bow etc), but also gives the opportunity in a tightly-packed convoy to hit more than one ship with a single blow.

So reload time, or number of reloads, doesn't really come into it, in terms of how the submarine engages in a single attack. They don't hang around to 'fight', if they can avoid it. Having done as much damage as possible, they then try to dive deep, hide beneath a different layer of water temperature or salinity (which makes them harder to find), and hope to slip away before they are located.

Where the number of reloads does make a difference is in the number of convoy attacks that a submarine can make during one deployment; but that was limited as much by its endurance - its ability to carry fuel, food and supplies, and the human endurance of submarine crews to remain effective for long periods of time in very harsh conditions - as by the number of torpedoes carried.

Now, a few further points. What to do when there are no more torpedoes? Go home. At least, that's what German submarines would do (they did have deck guns, and could attack unarmored lone merchantmen in that way, but the convoy system really rendered a gun attack by a U-boat on an allied convoy very dangerous). British submarines, in different theatres (e.g. the Med) did sometimes use their deck guns for shore bombardment.

Did the allies know the U-boats only had limited torpedoes? Yes, because it was the same on allied submarines. Torpedoes are big. WW2 submarines are quite small: they can't carry many. But it does not take many torpedoes to cause a lot of damage.

TLDR: A U-boat doesn't win by having a sustained shoot-out; it wins by being undetected, firing a successful salvo of torpedoes against a convoy, and escaping. One successful attack like that has justified the submarine's entire mission.

Sources:

Dimbleby, J. (2016) The Battle of the Atlantic: How the Allies Won the War.

Burn, A. (1998) Fighting Captain: The Story of Frederic John Walker RN, CB, DSO and the Battle of the Atlantic.

WWII submarines, including U-boats, had very different capabilities and filled a much different role compared to modern submarines. A WWII era submarine is best seen as mostly a low observability vessel (while cruising on the surface) with the ability to submerge for a short period (compared to modern nuclear powered submarines which can do cruises of several months fully submerged). The MO of a WWII era submarine is to cruise around on the surface looking for targets (which you can see long before they can see you, ideally), then submerge and attack, then get away. They are ambush predators, almost guerrilla warfare instruments, their main methods of dealing with extended combat with other armed vessels is just to hide below the surface and leave, they aren't meant for extended engagements.

However, when engaging much smaller or less well armed vessels (a good example would be landing craft, but could also include unescorted merchant vessels) they could use the deck gun.

Sometimes the subs (German or American) would prowl in "wolfpacks" so they could take on larger convoys without completely running out of torpedoes, however these had mixed results depending on the stage of the war (partly because they required lots of radio chatter which increased observability of the boats). However, most of the time on any given U-boat patrol the typical experience was that the U-boat didn't sink any vessels. When they did fire or sink a vessel it was usually just one during a patrol. It was very unusual for a U-boat to sink multiple vessels in a single patrol. And in that context, the amount of torpedoes carried starts to make more sense. Most U-boats didn't exhaust their torpedoes on any of their patrols, even during the "happy times" where U-boats sank a tremendous number of vessels.

Additionally, for the U-boat in particular the Germans created a way to resupply U-boats in the field (using Type XIV or "milk cow" U-boats), which enabled boats to have extended patrols, especially in areas far from German occupied ports.

The Allies knew that the U-boats only carried a small number of torpedoes, the US boats during the era carried a few more torpedoes (24 for the most common submarine classes). Perhaps the main way this short amount of resources affected anti-submarine warfare was that the Allies put a high priority on taking out those "milk cow" resupply submarines, though not specifically in relation to the low number of torpedoes carried by U-boats (fuel and food were also in short supply on a typical U-boat).