The number of targets chosen for Pyrotechnics must be at least one and at most four. You choose how the damage is divided as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.

If some but not all of Pyrotechnics’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 Pyrotechnics. 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 Pyrotechnics, the number of targets and division of damage can’t be changed. The effect that creates the copy may allow you to change the targets, however.