Charix, the Raging Isle — ruling 1

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases (such as that of Charix), then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Charix, the Raging Isle — ruling 2

An opponent's spell that targets Charix more than once costs only {2} more to cast.

Charix, the Raging Isle — ruling 3

Each time you activate Charix's last ability, the value of X for that instance of the ability is determined only as that ability resolves. Once that happens, the size of the power-and-toughness change for that instance of the ability won't change later in the turn, even if the number of Islands you control does change.

Charix, the Raging Isle : règles, fonctionnement et interactions (MTG)

Charix, the Raging Isle 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

Spells your opponents cast that target Charix cost more to cast. : Charix gets +X/-X until end of turn, where X is the number of Islands you control.

How it works : Charix, the Raging Isle

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2020· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases (such as that of Charix), then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#2September 25, 2020· Wizards of the Coast
An opponent's spell that targets Charix more than once costs only more to cast.
No other card is named in this ruling.
#3September 25, 2020· Wizards of the Coast
Each time you activate Charix's last ability, the value of X for that instance of the ability is determined only as that ability resolves. Once that happens, the size of the power-and-toughness change for that instance of the ability won't change later in the turn, even if the number of Islands you control does change.
No other card is named in this ruling.