Whenever a creature you control attacks, put a charge counter on Druids' Repository.
Remove a charge counter from Druids' Repository: Add one mana of any color.
The first ability of Druids’ Repository will trigger once for each creature you control that attacks.
You can activate the second ability of Druids’ Repository multiple times in the same turn. It’s a mana ability, doesn’t use the stack, and can’t be responded to.