If you control more than one, each time you take damage you can decide which one replaces the damage, and that one replaces all of the damage. You can't split the damage between them. You decide each time you take damage. Note that damage from multiple sources that happens at one time (such as combat damage) gives one choice, not one per source.

If you take 6 damage at once, it will replace all 6 damage and put 6 counters on it. Then the second ability will trigger. Any additional damage you take before this ability resolves also results in counters, but once the ability resolves, this card is sacrificed.

The effect does not prevent damage, it replaces it so the damage is neither dealt or prevented.