If multiple creatures would deal combat damage to you and your creatures at the same time, this prevents X of the damage that would be dealt by each of them. This prevention is applied creature by creature. For example, if Cover of Winter has two age counters and you're being attacked by an unblocked 1/1 and an unblocked 4/4, this will prevent 2 of the combat damage that would be dealt by each one. All damage from the 1/1 is prevented and 2 damage from the 4/4 is prevented, meaning you're dealt 2 damage.