Text (SM+errata): Whenever a -1/-1 counter is placed on a creature, you may put a 1/1 green Elf Warrior creature token onto the battlefield. [Oracle 2009/07/08]
The ability triggers when a -1/-1 counter is placed on a creature in play, and when a creature enters play with a -1/-1 counter on it (as with the Persist ability). [Shadowmoor FAQ 2008/04/21]
It triggers once for each -1/-1 counter. [Shadowmoor FAQ 2008/04/21]
