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 abilities are now written as "amass [subtype] N." Previous cards with amass have received errata to say "amass Zombies N."

You don't choose a target for Grishnákh, Brash Instigator's ability at the time it triggers. Rather, a second "reflexive" ability triggers when you amass Orcs this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

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.

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 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.

Amass Zombies works the same way, except you create a 0/0 black Zombie Army creature token if you don't control an Army. If the Army creature you chose isn't already a Zombie, it becomes a Zombie in addition to its other types. By combining cards with amass Orcs and amass Zombies, you can end up with an Orc Zombie Army.

To amass Orcs N, if you don't control an Army creature, create a 0/0 black Orc 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 an Orc, it becomes an Orc in addition to its other types.