Chaos Moon — ruling 1

In Two-Headed Giant, triggers only once per upkeep, not once for each player.

Chaos Moon : règles, fonctionnement et interactions (MTG)

Chaos Moon is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of each upkeep, count the number of permanents. If the number is odd, until end of turn, red creatures get +1/+1 and whenever a player taps a Mountain for mana, that player adds an additional . If the number is even, until end of turn, red creatures get -1/-1 and if a player taps a Mountain for mana, that Mountain produces colorless mana instead of any other type.

How it works : Chaos Moon

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2006· Wizards of the Coast
In Two-Headed Giant, triggers only once per upkeep, not once for each player.
No other card is named in this ruling.