I'm curious, what brought you to the idea that the number of creatures an opponent controls should be the determining factor in the damage done? I don't dislike the idea, it is simply a bit different (to someone who doesn't browse the CI&A boards much) and I'm not quite sure if it fits
or could possibly be tweaked thematically. Balance-wise, it certainly doesn't feel OP to me, and probably even UP, considering most times you don't have more than 6-8 creatures out in a game unless of course Fractal is involved. For that reason, I would consider either reducing the costs or changing the damage mechanism (maybe 5+X damage, or the like?).