Many triggered abilities trigger when an attacking creature becomes blocked. However some of them trigger just once regardless of the number of creatures that are blocking the attacking creature and others trigger once per each blocking creature. The difference is in how the trigger condition is written.

Abilities that say ‘Whenever ~this creature~ becomes blocked’ refer to the moment when the attacking creature becomes a blocked creature. Because of this the ability will only trigger once regardless of the number of creatures that are blocking it. Examples of cards with this type of ability would be Ichorclaw Myr and Escaped Null. For example, an attacking Ichoclaw Myr will only get +2/+2 when it becomes blocked, even if it is blocked by more than one creature.

Abilities that use the phrasing, ‘Whenever ~this creature~ becomes blocked by a creature’ refer to each creature that ends up blocking this attacking creature. Because this ability cares about the individual creatures that are blocking, it will trigger once for each blocking creature. Examples of cards with abilities worded this way include Engulfing Slagwurm and Infiltration Lens. For example, if an attacking creature that is equipped by Infiltration Lens is blocked by two separate creatures, the controller of this attacking creature will end up drawing four cards. (Technically speaking, there are two triggers so they will draw two cards when the first trigger resolves and then another two when the other trigger resolves.)