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If you don't control an Army, the Goblin 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.
In the rare case that you control multiple Army creatures (perhaps because you control a creature with changeling) while you amass Goblins, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Goblin, it becomes a Goblin in addition to its other types.
Some spells and abilities that amass Goblins 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 Goblins.
To amass Goblins N, if you don't control an Army creature, create a 0/0 black Goblin Army creature token. Then you choose an Army creature you control and put N +1/+1 counters on it. If that Army isn't already a Goblin, it becomes a Goblin in addition to its other types.
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.
Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. And if the Army creature you chose isn't already an Orc, it becomes an Orc in addition to its other types. By combining cards with amass Orcs and amass Goblins, you can end up with a Goblin Orc Army.
