Kurbis enters with a number of +1/+1 counters on it equal to the total mana spent to cast it, and that number usually won't be the same as X. If you cast Kurbis for {G}{G}, it will enter the battlefield with two +1/+1 counters on it.

If you copy Kurbis while it is on the stack (perhaps using an effect like that of Double Major), the copy wasn't cast, so no mana was spent to cast it. That Kurbis will enter with zero +1/+1 counters on it.

Similarly, if you create a token that's a copy of Kurbis, or if a card enters the battlefield as a copy of Kurbis without being cast, it enters with zero +1/+1 counters on it.

If a resolving spell enters the battlefield as a copy of Kurbis, it will enter the battlefield with a number of +1/+1 counters equal to the mana spent to cast the spell. For example, if you spend {3}{U} to cast Clone and choose to have it copy Kurbis as it enters the battlefield, it will enter with four +1/+1 counters on it because four mana was spent to cast Clone.

Once the activated ability resolves, damage will be prevented even if the creature later loses all of its +1/+1 counters.