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.

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

Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. 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 Zombies, you can end up with an Orc Zombie Army.

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.

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.

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

If there are no instant or sorcery cards in your graveyard, you'll amass 0. If you don't control an Army, you'll create a 0/0 black Zombie Army creature token that will die after Invade the City has finished resolving.

The number of instant and sorcery cards in your graveyard is counted only as Invade the City resolves.

A split card that's both an instant and a sorcery is counted only once for Invade the City.

Invade the City is still on the stack while you count your instant and sorcery cards in your graveyard. It doesn't count itself.