You choose how the damage will be divided among the target creatures at the time you cast Hail of Arrows. Each target must be dealt at least 1 damage. If any of those creatures becomes an illegal target before Hail of Arrows resolves, the division of damage among the remaining creatures doesn’t change.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.