You put only one +1/+1 counter on Ironscale Hydra per creature whose combat damage is prevented, no matter how much damage would have been dealt.
2020-01-24
If damage that can’t be prevented is dealt to Ironscale Hydra, the damage is dealt and you’ll simultaneously put a +1/+1 counter on it, before the game checks for lethal damage.
If a creature would deal combat damage to this creature, prevent that damage and put a +1/+1 counter on this creature.
