Sphinx's Windfall

: Target opponent reveals a card at random from their hand. If it's an instant or sorcery card, exile it. You may cast it without paying its mana cost for as long as it remains exiled. At the beginning of the next end step, if you haven't cast it, return it to its owner's hand. Activate only as a sorcery.

Each player discards their hand, then draws cards equal to the greatest number of cards a player discarded this way.