Each AMM-firecon can only engage a single enemy salvo per 5-second tic. Assigning a lot of launchers to a single firecon works fine, if the enemy is using only a few, but large salvos, but if you are facing a swarm of enemy FACs for example, you will be swamped with many small salvos. 3 to 5 launchers per firecon is thus more flexible as several firecons will launch vs. each large salvo, while each firecon will engage a seperate salvo if you are attacked by many small ones.
Automatic PD is set-up on the Battle Control window (F8)
Select the firecon, assign launchers and missiles, then set the point defense mode and hit "Set Mode" - Done
Beware! If you have both, beam and missile firecons and the beam firecons are listet before your missile firecons, the Max. PD Range will default to the range of your beam firecon. CHANGE THIS TO THE RANGE OF YOUR MISSILE FIRECON! Otherwise, your AMMs will be launched only at beam ranges.
Generally, go max power with missile engines, forget fuel efficiency.