Ironscale Hydra — ruling 1

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.

Ironscale Hydra — ruling 2

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.

Ironscale Hydra : règles, fonctionnement et interactions (MTG)

Ironscale Hydra is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

If a creature would deal combat damage to this creature, prevent that damage and put a +1/+1 counter on this creature.

How it works : Ironscale Hydra

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
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.
No other card is named in this ruling.