Well, I once encountered both an anti-missile Precursor ship and one of their PCDs. I got hit by more than 2500 missiles. Just to give you an idea.
Second, as I hinted to earlier, these are anti-missile missiles, and they seem to act quite opportunistically. The Prix don't know if we fire missiles at them, they don't have long distance stuff, so they launch their size-1 missiles preemptively. I have the feeling they hit the first enemy target they meet.
This means that if you can maintain a fire of 1, 2X1 or 3X1 missiles (ideally size-1 missiles, the most expendable and cheapest) going straight beyond your most advanced ships, the entire 19X1 missile salvos will hit your lone missile and go to waste.
In your current situation, they're probably out of range. In which case, the best solution is probably to designate a waypoint some 2-3m km beyond the Rocks and fighters, fire your own anti-missile at them, one at a time, and hope the Prix missiles will go for the decoys.