Whatever you set it, it is 1, 2, 3 or 4 vs. each enemy missile, not salvo.
Standard settings seem to be 2 or 3 vs. 1 until you know more about the enemy. If, for example, you have a tech advantage, your AMMs should have an easy time intercepting the enemies ASMs, so a 1 vs. 1 setting might be in order to not waste a lot of AMMs.
If it´s the other way around, you will probably have to go to 4 vs. 1 and still not be able to take out all of the enemy missiles.
Note: Look at the speed of the enemy missiles. Compare this to the intercept chances of your missiles and it will give you an idea as to what the correct setting might be.
I do belive, it´s a streight progression, i.e. if your missiles have a 200% chance to intercept a speed 10k missile, they will have a 100% chance to intercept a 20k missile and a 50% chance to intercept a 40k missile (I am not absolutely sure on this, so use this info at your own risk
)