509.4c An ability that reads “Whenever [this creature] becomes blocked, . . .” generally triggers
only once each combat for that creature, even if it’s blocked by multiple creatures. It will trigger
if that creature becomes blocked by at least one creature declared as a blocker. It will also
trigger if that creature becomes blocked by an effect or by a creature that’s put onto the
battlefield as a blocker, but only if the attacking creature was an unblocked creature at that time.
(See rule 509.1h.)

509.4d An ability that reads “Whenever [this creature] becomes blocked by a creature, . . .” triggers
once for each creature that blocks the named creature. It triggers if a creature is declared as a
blocker for the attacking creature. It will also trigger if an effect causes a creature to block the
attacking creature, but only if it wasn’t already blocking that attacking creature at that time. In
addition, it will trigger if a creature is put onto the battlefield blocking that creature. It won’t
trigger if the creature becomes blocked by an effect rather than a creature.

509.4e If an ability triggers when a creature blocks or becomes blocked by a particular number of
creatures, the ability triggers if the creature blocks or is blocked by that many creatures when
blockers are declared. Effects that add or remove blockers can also cause such abilities to
trigger. This applies to abilities that trigger on a creature blocking or being blocked by at least a
certain number of creatures as well.