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

Each token copies exactly what is printed on the creature and nothing else (unless that creature is copying something else; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on.

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

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

If the entering creature leaves the battlefield before Mirror March’s ability resolves, use that creature’s last known existence on the battlefield to determine the token’s characteristics.

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.