The number of targets chosen for Boulderfall must be at least one and at most five. You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.

If some but not all of Boulderfall’s targets become illegal, you can’t change the division of damage. Damage that would have been dealt to illegal targets simply isn’t dealt.

You can’t deal damage to both a player and a planeswalker that player controls using Boulderfall. You also can’t deal damage to more than one planeswalker controlled by the same player. If you choose to redirect the damage being dealt to a player to a planeswalker, you must redirect all the damage to a single planeswalker.

If an effect creates a copy of Boulderfall, the division of damage and number of targets can’t be changed. The effect that creates the copy may allow you to change targets, however.