If a creature says "whenever [this creature] becomes blocked" and is blocked by multiple creatures, how many times will it trigger?

When a creature has an ability that triggers "whenever [this creature] becomes blocked," that ability will activate one time each time the creature is blocked, regardless of how many creatures are blocking it. This is because the ability is not based on the number of blockers but rather on the specific event of the creature becoming blocked.

When a creature becomes blocked by multiple creatures, the triggering event is still considered a single instance—allowing the ability to trigger once. The rules regarding triggered abilities clarify that this scenario results in just one trigger, even if several creatures block the attacker.

Thus, the correct understanding is that the trigger signifies the event of being blocked, not how many blockers there are. An ability that states it triggers "whenever [this creature] becomes blocked" will only activate once no matter how many blockers are present.