Arboreal Alliance

• WWizards of the Coast2023-11-03
  Official Ruling
  Arboreal Alliance's last ability triggers whenever you attack with at least one Elf. You populate exactly once as it resolves, no matter how many Elves you attacked with.
• WWizards of the Coast2023-11-03
  Official Ruling
  If Arboreal Alliance enters the battlefield without being cast, or if you cast it with X equal to 0, you'll create a 0/0 green Treefolk creature token. Unless something else is raising its toughness, it will immediately die.
• WWizards of the Coast2024-01-12
  Official Ruling
  If you choose to copy a creature token that's a copy of another creature, the new creature token will copy the characteristics of whatever the original token is copying.
• WWizards of the Coast2024-01-12
  Official Ruling
  Populate doesn't target the creature token you're copying. You choose that creature token as you're taking the populate action. You can choose any creature token you control. If a spell or ability causes you to create a creature token and then instructs you to populate, you may choose to copy the token you just created, or you may choose to copy another creature token you control.
• WWizards of the Coast2024-01-12
  Official Ruling
  If you control no creature tokens when you populate, nothing will happen.
• WWizards of the Coast2024-01-12
  Official Ruling
  Any enters-the-battlefield abilities of the copied token will trigger when the new token enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the copied token will also work.
• WWizards of the Coast2024-01-12
  Official Ruling
  The new creature token copies the characteristics of the original token as stated by the effect that created the original token.
• WWizards of the Coast2024-01-12
  Official Ruling
  The new token doesn't copy whether the original token is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any noncopy effects that have changed its power, toughness, color, and so on.

Rulings via Scryfall