In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Zombies, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Zombie, it becomes a Zombie in addition to its other types.
Commence the Endgame
If you don't control an Army, the Zombie Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.
Some cards refer to the "amassed Army." That means the Army creature you chose to receive counters, even if no counters were placed on it for some reason.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Noah Bradley
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
