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

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.

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.