As with everything in Aurora, it depends
Every Firecon can only engage one single target (in that case, a salvo of enemy missiles) per 5 second increment.
If you are facing relatively few, but large salvos of incoming ASMs, few or even only one AMM-Firecon will be sufficient.
If you are facing many, small salvos, on the other hand, you will need many AMM-Firecons, or some of those salvos will get _very_ close before they get targetet by AMMs.