Per the spell, they follow spoken commands of the animator. It is a free action to speak, so free.
No, I would not make it a longer action with more undead, they would simply have to be within earshot.
As an additional note, even though you are allowed any number of free actions in a round, you are limited to a few sentences. As GM I'd allow 3 separate commands in a single round (without limiting the size of the groups commanded).
Allowing more commands than that would seem to be a bit overboard for a single round of speaking. Not only that, it would be rather unlikely more would ever be necessary.