I was looking at making a Protector Aasimar Sorcerer and I read the Radiant Soul ability adding extra radiant damage equal to your level. I’m wondering for a spell like Fireball does it effect all creatures caught in the blast or for just one creature similar to the Evocation Wizard’s Empowered Evocation.

It does what it says.

Here you go:

[...] once on each of your turns, you can deal extra radiant damage to one target when you deal damage to it with an attack or a spell.

Just one creature on each of your turns.

"One target" does not mean "one damage roll" like what empowered evocation has.

Thanks guys. I missed that part