both
When prepared and served as food, it is usually uncountable.
"We'd eaten fish for dinner that night."
"How MUCH fish did you eat?"
"A LOT of it!"
Often that is the case, as when it is food we treat it as a substance, unless you actually knew how many animals it was before eaten!
I'd be surprised if you heard people counting how many they ate... "we ate 3 fish!"; I'd assume you ate them while they were still alive! Haha... Because:
As an animal, they can be counted; however, it uses an unchanged plural (the plural and singular are in fact identical).
1 fish
2 fish
However, it is not uncommon for English speakers to make a mistake and say "fishes", but it is not generally accepted and is incorrect.