When this enchantment enters and whenever an Army you control attacks,
amassTo amass [subtype] N means “If you don’t control an Army creature, create a 0/0 black [subtype] Army creature token. Choose an Army creature you control. Put N +1/+1 counters on that creature. If it isn’t a [subtype], it becomes a [subtype] in addition to its other types.”
A player “amassed” after the process described in rule 701.47a is complete, even if some or all of those actions were impossible.
The phrases “the Army you amassed” and “the amassed Army” refer to the creature you chose, whether or not it received counters.
Some older cards were printed with amass N without including a subtype. Those cards have received errata in the Oracle card reference so that they read “amass Zombies N.” (CR 701.47) Orcs 1. (Put a +1/+1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on an Army you control. It's also an Orc. If you don't control an Army,
createTo create one or more tokens with certain characteristics, put the specified number of tokens with the specified characteristics onto the battlefield.
If a replacement effect applies to a token being created, that effect applies before considering any continuous effects that will modify the characteristics of that token. If a replacement effect applies to a token entering the battlefield, that effect applies after considering any continuous effects that will modify the characteristics of that token.
Previously, an effect that created tokens instructed a player to “put [those tokens] onto the battlefield.” Cards that were printed with that text have received errata in the Oracle card reference so they now “create” those tokens. (CR 701.7) a 0/0 black Orc Army creature token first.)