Very simple and very effective:
1-6 targets, 100% dmg to all targets
7 targets, 90% dmg to all targets
8 targets, 80% dmg to all targets
9 targets, 70% dmg to all targets
10 targets, 60% dmg to all targets
11 targets, 50% dmg to all targets
12 targets, 40% dmg to all targets
12++ targets, 30% dmg to all targets
This will we much more realistic than a hard cap.