I think the crucial missing information here is the size of the Salvo.
Regardless of whether you choose to allocate 2, 3, 5, or any other number of launchers to Fire Controls, it's imperative to consider the size of the Salvo. Dealing with a 50 missiles Salvo is vastly different from handling just a 5 missiles Salvo.
Expanding the discussion, relying solely on AMMs as a PD strategy is excessively costly.
In my opinion, effective PD demands a layered system incorporating AMMs, decoys, and possibly turreted PD, or CWIS.
Back on topic, if the salvos are arriving simultaneously within the same 5-second interval, then you'll require 1 FC system per salvo. You could also opt to assign 1 escort ship per salvo rather than one ship for multiple salvos, and retain only 1 additional FC for redundancy (or split the barrage into 2) while reallocating the saved tonnage to incorporate more launchers.