If multiple nontoken creatures are put into their owners' graveyards from the battlefield at the same time, the imprint ability will trigger that many times. You put the triggered abilities on the stack in any order, so you'll determine in which order they resolve. However, since exiling those cards is optional, and choosing to exile a card this way causes the previously exiled cards to return to their owners' graveyards, the order generally doesn't matter: You'll wind up with at most one of those cards exiled, and the rest will be in the appropriate graveyards.