The animate dead spell has a range of touch and instantaneous duration and requires gems to be placed in the mouths or eye sockets of all of the corpses effected by the spell. (see below)
Targets: One or more corpses touched
This spell turns the bones or bodies of dead creatures into undead skeletons or zombies that follow your spoken commands.
Material Component You must place a black onyx gem worth at least 25 gp per Hit Die of the undead into the mouth or eye socket of each corpse you intend to animate. The magic of the spell turns these gems into worthless, burned-out shells.
How is one supposed to animate more than one undead at a time with this spell if it has an instantaneous duration and a touch range including placing the gems?