Marath, Will of the Wild enters the battlefield with a number of +1/+1 counters on it equal to the amount of mana spent to cast it.
X, Remove X +1/+1 counters from Marath: Choose one —
• Put X +1/+1 counters on target creature. X can't be 0.
• Marath deals X damage to any target. X can't be 0.
• Create an X/X green Elemental creature token. X can't be 0.
10/17/2013 You announce the value of X as you activate the ability, and all instances of X in the activation cost are equal to the announced value. For example, if you choose 2 as the value of X, then you pay and remove two +1/+1 counters to pay the cost.
2/1/2014 The amount of mana you spent to cast this creature is usually equal to its converted mana cost. However, you also include any additional costs you pay, including the cost imposed for casting your commander from the command zone.
2/1/2014 You can’t choose to pay extra mana to cast a creature spell unless something instructs you to.
2/1/2014 If Marath enters the battlefield without being cast, then no mana was spent to cast it. It will therefore enter the battlefield without any +1/+1 counters. If no other effects are increasing its toughness at that time, it will subsequently be put into its owner’s graveyard as a state-based action.