Chronicidal and Alexander's answers combine to be the winner.
In the back and forth of nuclear war, one thing the US tried was the tight-pack. Earlier, they had tried spreading silos far apart, forcing the enemy to counter with 1 missile per silo, which the enemy promptly did, set up to arrive time-on-target, or all together to overwhelm any ABM systems.
So the US threw the "Time on Target" strategy right back in their face. With the tight-pack, the US put them close together - hardened enough that all but the direct-hit target would survive. But close enough, that a time-on-target attack would ensure that the first missile would cause fratricide -- all the subsequent missiles would slam into the shockwave of the first missile, and be damaged sufficiently to not detonate.
So in your case, it wouldn't be shockwave, it would be something like shrapnel - suppose you had interceptor missiles of roughly equal warhead
velocity x mass to the attacking missiles, so their collision would cancel each other's velocity before causing a massive explosion of sharpnel to occur relatively stationary in space. This creates a localized Kessler Syndrome between attacker and defender, then causing fratricide of a salvo of any size -- the bigger the salvo, the bigger the Kessler Syndrome.
The ship would stock two kinds of interceptors.
- small, designed to break an incoming missile so its attack would be ineffective, but it would do little to change the velocity vector of the incoming missile and its soon to be shrapnel bits; those would continue on past the defender at roughly equivalent speed, and be dealt with by the defender's normal micrometeorite shield. These were numerous but its launch rails or targeting could be overwhelmed by a salvo. So... they also have
- Large, designed to match the
mass x velocity of an attacking missile, and create this "Kessler syndrome" field of debris. These missiles are more precious, and would be reserved for salvos large enough to overwhelm the first defense.
As such, attackers find that the most effective attack is a "happy medium" - too small a salvo to justify trotting out the Kessler defense weapon, yet large enough to get a couple past the single defenses if you get lucky. So you have numbers of such salvos until you get lucky.
See also Hellfire vs Arena. Too small a salvo (1) is efficiently dealt with by the Arena defense system. Too big a salvo, well, it would certainly squish the tank, but would be totally unmanageable from the attacker's end, and would run you out of ammo, leaving you unable to attack other tanks. Generally it's a lose if the ammo you throw at the tank costs more than the tank.