When ships fire one ASW torpedo and when they salvo

Are anti-submarine torpedo tubes fired all at once or one weapon at a time?

Usually one at a time during a controlled prosecution, but a ship under attack or engaging at long range may fire multiple weapons, often with a short interval.

A ship prosecuting a submarine contact without coming under attack would normally release one torpedo at a time. A modern ASW weapon does not usually require a spread, and forcing the submarine to move can make it easier to track. If hostile torpedoes are already incoming, the response can be a snapshot down the contact bearing with every tube that can bear. The one-weapon preference applies to an individual asset: several ships or aircraft pursuing the same submarine may release weapons at similar times. Aircraft or rockets can deliver weapons at longer range, but their travel time delays a follow-up attack. Rocket-delivered torpedoes may therefore be launched in a staggered pair, with the interval adjusted to the engagement.