Book of Mazarbul

If you don't control an Army, the Orc 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.

Some spells and abilities that amass Orcs may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't amass Orcs.

In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Orcs, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't an Orc, it becomes an Orc in addition to its other types.