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.

You can cast Treason of Isengard without a target just to amass Orcs. However, if you do choose a target, and that target is illegal at the time Treason of Isengard tries to resolve, Treason of Isengard won't resolve and none of its effects will happen. 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.

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

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.

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.

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.