You divide the prevention effects as you cast Remedy, not as it resolves. Each target must be assigned at least 1 prevention “shield.” You can’t choose more than five targets and assign none to a target.

If some of the targets become illegal, the original division of prevention effects still applies, but the effects that would have been created for illegal targets aren’t created at all.

If multiple sources would deal damage to an affected target at once, that target or that target’s controller chooses which source’s damage to prevent for each “shield.”

If multiple replacement effects apply to a player or permanent being dealt damage, that player or the controller of that permanent chooses the order to apply them, not the controller of the source of damage.