Soft cap abilities have what's called square root scaling, meaning if you hit 2 times the soft cap worth of targets, you do root 2 times the total damage.

Here's an example on a 5 target capped move that does 200 damage per target.

1 target: 200 damage per target/200 damage total
2 targets: 200 damage per target/400 damage total 3 targets: 200 damage per target/600 damage total 4 targets: 200 damage per target/800 damage total 5 targets: 200 damage per target/1000 damage total 6 targets: 183 damage per target/1098 damage total 7 targets: 169 damage per target/1183 damage total 8 targets: 158 damage per target/1264 damage total 9 targets: 149 damage per target/1341 damage total 10 targets: 141 damage per target/1410 damage total

So past the soft cap, each additional target is worth less than the last, but you will still always gain damage by having more targets.