Alright, since you are EV training for attack in your scenario, I will use Shuppet as an example since it has one point for the attack stat per Shuppet fainted. With the Power Bracer held, four points will be added to the one point that each Shuppet gives off, leading to five points per Shuppet in that stat. If you Pokémon has Pokérus, the end result will be doubled, leading to ten points gained in attack per Shuppet. With this information, a Horde of Shuppets will give you 25 EV's for a Pokémon with a Power Bracer defeating a Shuppet Horde, since a horde will have five Shuppets. If it has Pokérus, 50 Attack EV's will be gained in attack per horde. If you were to fight a Vulpix horde (1 EV Point in Speed), you would gain one point in speed and four points in attack per Vulpix, and two points in speed and eight points in attack with Pokérus. That should give you a basic sense of how a Power Bracer works with a horde.