If a creature has multiple instances of afflict, each triggers separately.

To amass Slivers 2, if you don't control an Army creature, create a 0/0 black Sliver Army creature token. Then you choose an Army creature you control and put 2 +1/+1 counters on it. If that Army isn't already a Sliver, it becomes a Sliver in addition to its other types.

If you don't control an Army, the Sliver Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.

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.

In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Slivers, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Sliver, it becomes a Sliver in addition to its other types.