If multiple creatures block a creature with afflict, afflict triggers only once.

Afflict causes the defending player to lose life; it's not damage or combat damage.

Afflict resolves before combat damage is dealt. If this loss of life brings a player to 0 life or less, that player loses the game immediately. A blocking creature with lifelink won't deal combat damage in time to save that player.

If a creature is attacking a planeswalker, that planeswalker's controller is the defending player. If a creature is attacking a battle, that battle's protector is the defending player.