Spreading Flames deals 6 damage divided as you choose among any number of target creatures.
You announce how the damage will be divided as part of casting Spreading Flames. Each chosen target must receive at least 1 damage.
If Spreading Flames has multiple targets, and some, but not all, are illegal targets when Spreading Flames tries to resolve, Spreading Flames will still deal damage to the remaining legal targets according to the original damage division. If all the targets are illegal, the spell won’t resolve.