When using Drampa GX's "Berserk" attack, must ALL Benched Pokemon have at least 1 damage counter or do you only need 1 Benched Pokemon with a damage counter on it to add the +70 damage?

If any of your Benched Pokemon have damage counters it will add the +70 damage. If it required all of them it would have said "all" or "each" in the attack text.

Relevant cards

SMA_SV80