SE4 does'nt handle ammo ... basically it's infinite, but having a reload time of 3 rounds and battles Lasting 30 turns, your ships have in fact 10 missiles/component, with automatic replenishment after battle !
In DevNull/Derek mod there are also "External racks" with reload time of 30, so in fact basically one shot weapons.
As a SFC player, that bothers me but is not totally "unrealistic"...