Given that we're not wargaming, I don't see why we need to know how many missiles a ship is launching except as a ratio of the total number of missiles it carries (so that we know when it runs out of ammunition.) What we really need to know is how many missiles reach their target. The equation 'how many of a ship's missiles reach their target?' depends on ECM, ECCM, maneauvering, point defense, and any other ability to deflect or escape from incoming attacks.
Point defense, for example, is capable of defeating anywhere from 0 to over 99% of incoming missiles depending on various factors. So, the number of missiles a ship fires is immaterial, unless the target has no defenses and circumstances for the attack are ideal, as when bombarding a defenseless planet. In that case, the damage caused, whether with missiles or with directed energy weapons, is best described as 'as much as the attacker wants'.
In any other case, a ship that launches hundreds of missiles should still be expected to land a reasonable number of hits to be practical for the RP, not dozens of times more than are needed to kill the target. The issues we're dealing with don't come up unless we're trying to decide, based on the design and description of the launcher, the missiles, and the targets, what the hit percentage will be, and not as GMs but as simulationists.
I don't think we should go down this road, it leads to a potential for ten thousand pages of arguments and all we'd get at the end would be a constantly evolving space missile combat simulation.