I fire seven beams, and a randomly determined number strike you.
Hmm. It's an interesting one, because if I fire three Scorching Rays, a variable number might strike you depending on my attack rolls, and I wouldn't consider the number striking you to be a variable numeric effect in Scorching Ray's case.
And yet in Prismatic Spray's case, where every creature in the area is struck by a randomly-determined number of beams, I do. Similarly, in the case of the old Quill Blast (back when Large creatures were struck by 4d6 quills), for example, I'd consider the number of quills striking the creature to be Empowerable.