When Volibear is on board and an opponent moves multiple units to a battlefield in one action, do you draw once or once per unit moved?

Ruling: You draw only once when an opponent moves multiple units to a battlefield in a single action.

Nuances:

• Volibear's trigger is based on "when an opponent" takes an action, not "when an enemy unit" moves
• If the card said "when an enemy unit" instead, you would draw for each individual unit moved