That's right. If they are dead, shoot once. If they almost dead, shoot twice. If they are still alive, empty your ammo. All of these just to make sure they are DEAD.
Since the handguns are revolvers, if she needs just one blessed Milkor MGL (the grenade launcher), it stands to reason to have many revolvers, since those take a while to reload, even with speedloaders/half-moon clips/full moon clips. And since I don't see any pockets on that dress, it's safe to assume she has no reloads, ergo she throws away the guns as they empty.
Even so she might run out of ammo: six rounds per revolver with four revolvers (eight if they continue around her waist), one per musket, two more in the flintlock pistols, four more in the flintlocks in the white holsters (eight if she has four more on the other side of her waist). This gives her 33-37-57 rounds, plus six grenades in the Holy Milkor before she has to switch to the bayonet (which would be the first to be fired and discarded in the event of a firefight, since it doesn't appear to have a holster).