Mirror-Style Master

All of the tokens enter the battlefield at the same time.

Although the tokens enter the battlefield attacking, they were never declared as attacking creatures (for the purposes of abilities that trigger whenever a creature attacks, for example).

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the copied creature will also work.

Backup confers only abilities that are actually printed below it. Any abilities that are gained by the permanent are ignored, including abilities gained due to a resolving spell or ability or copy effects.

Each of the tokens copy exactly what was printed on the original creature and nothing else (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on. (Basically, it won’t copy anything that made it modified in the first place. C’est la vie.)

If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.

If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.

If a permanent enters the battlefield as a copy of a card with a backup ability or a token is created that is a copy of that card, the order of the printed abilities is maintained.

If the copied creature has {X} in its mana cost, X is 0.

If the copied creature is a token, the token that’s created copies the original characteristics of that token as stated by the effect that created that token.

If the copied creature is copying something else, then the token enters the battlefield as whatever that creature copied.

The abilities that backup grants to the target creature are determined only once, at the time the ability triggers. They won’t change if the permanent with backup loses any abilities before the backup ability resolves.