Note that it isn't the number of missiles themselves that determines damage, but the effect of the attack roll for the salvo (core rulebook 162). Resolve the damage as normal fora single missile (so roll 4D, subtract armor), then multiply by the effect of the roll. Each missile adds a +1 DM to the attack roll, so a 120 missile salvo adds +120 to the attack roll. Assuming no other modifiers beyond the die roll (result of 2-12), that gives a total of 122 to 132 on the attack roll. Subtract 8 to find the Effect, you get 114 to 124. So a single salvo of 120 missiles, assuming none are shot down or otherwise destroyed before they impact, could do anywhere from 114 to 124 times a single missiles damage.
Likewise, the 30 torpedoes add +30 on the attack roll, for a total Effect range of 24 to 34, resulting in 30 torpedoes dealing the average damage of 24 to 34 torpedoes.
That also means that a single missile, with a good attack roll (say, Effect 4), will do more than 1 missiles average damage. With Effect 4, a single missile will do 4x normal damage, or the average damage of 4 missiles.