At the beginning of your upkeep, exile a card from your graveyard.
Threshold — As long as seven or more cards are in your graveyard, this enchantment has "Whenever a nontoken creature is put into your graveyard from the battlefield, you may pay

. If you do, return that card to your hand."
Multiple creatures going to the graveyard at the same time all end up there at once, so either all of them or none of them will cause the trigger to go off, depending on whether or not Threshold was met prior to the creatures going to the graveyard.