The best way to balance grenades is to make them rare, really rare, so that players usually only use them in kind of emergencies and not spam them.
That's the absolute worst way to balance any 'consumable' item.
You end up with an
Elixir scenario: An item so rare(and powerful), that you never use it in case you need it more in the next fight.
Ideally, those items should be available for every fight(specifically because they are there to cover for weaknesses of your deck), especially if you make the effort to bolster your stock through crafting or purchase.
For the sake of consistency with the Memory deck system, there should be at the very least a soft cap on the number you can effectively deploy in a single fight(how many you can deploy in general is already soft-capped by the supplies you can find/buy).
