Mortal Combat

• WWizards of the Coast2007-07-15
  Official Ruling
  If you don’t have twenty or more creature cards in your graveyard by the time your upkeep starts, the ability won’t trigger that turn.
• WWizards of the Coast2007-07-15
  Official Ruling
  Mortal Combat’s ability has an “intervening ‘if’ clause,” so the condition is checked again when the triggered ability resolves. If you don’t still have twenty creature cards in your graveyard, the ability does nothing.

Rulings via Scryfall