In some offices that have to close down an account when a customer dies, the Zombie Army is the pile of paperwork that is waiting to be dealt with.
The reason for this is that the actual person is dead, but their account is still live until the necessary processing has been completed, meaning that they're dead, yet undead, hence a Zombie.
Since in my office, there are always large volumes of outstanding customer deaths to process, the vast numbers would enable them to be an army, should they decide to rise up!
"I have a huge Zombie Army that needs dealing with" (I've got loads of work to do)
"How many are in your Zombie Army" (How much outstanding work have you got?)