Given the fact that the ambassador had at least 8 tubes in total that were salvo capable quite easily, that fact is cannon from reading the lost era books by the way, its endorsed by paramount so its cannon.
I'd like to wind your memory back to nemesis how many torpedoes did the E fire? 10's of them and that was only against a single target, how about First Contact all ships fire 10's and yet again a single target. The new Star Trek Movie many torpedoes from several of the ships in film (Enterprise, Narrada) fired yet again at a single target. The Falmouth is designed to take on many targets at once. Each tube is only supposed to fire 2 each now consider what I said earlier it is designed to go up against the cardassian hunting packs of the era more or less on its own, that means at least 3 or 4 ships at once, so you spread your torps across the attacking wolf pack its simple logical and makes tactical sense, you knock out or down shields at a longer range with torps and then let the phasers do the rest when in close again the most logical and tactical move to make the enemy think twice before firing as they may hit their own when in close proximity mixed up in the pack, this too is the reason for large impulse engines. Rather than putting a salvo in one or two tubes which takes a long time to load and one direct hit and you entire forward or aft torpedo offensive ability is destroyed it is also laid out as such to enable longer lasting weapons placement meaning that the enemy has to target many areas to destroy her offensive ability again for anti wolf pack tactics if there are launchers it cant see in a direct line of sight, hence weapon arc shadow caused by the ship itself, then the cardy can't destroy your other launchers until it maneuvers through your other weapon arcs thus opening itself up for attack from your others arcs. Note also I said STVI type torpedoes not the current Enterprise C torpedoes that could would possinly be in use around this period, reasons easy to build from spares (older more understood technology) replicators are only now being introduced in this period therefore torps would have to be made from scratch if you didn't have replicators and secondly the more modern torps are designed for the more modern launchers in the ambassador class and onwards which use larger salvo's which the older style launchers couldn't. I request you cast your memory to ST6 both the excelsior and Enterprise only fired a maximum of 2 torpedoes from each tube before reloading. Hence the more torpedo tubes because the technology of the era wasn't capable of firing large torpedo salvos.
I have thought this through thoroughly and have played the devils advocate with the entire design so I have asked myself this question more than once but I can warrant the amount of torp tubes by the evidence provided above using the cannon examples and just evolving the amount of tubes and lessening the salvo amounts keeping the amount of torpedoes. Enterpise-D could fire up to 12 or 16 of her much more advanced and destructive photons from one tube FWD and one tube AFT if I remember.
