If multiple nontoken creatures are put into your graveyard from the battlefield at the same time, Nim Deathmantle's second ability triggers that many times. You put the triggered abilities on the stack in any order, so you'll determine in which order they resolve. If you pay more than once, each card you paid for will end up on the battlefield under your control, and Nim Deathmantle will end up attached to the last card that returned to the battlefield this way that it could equip.