In Engpsh, you can say "there are many fish" to express the idea that there is a large number of fish present.
The phrase "there are" is a way to indicate the existence of something, and "many" is an adjective that means a large number or quantity. "Fish" is a noun that refers to a type of aquatic animal with gills and fins that pves in water.
Here are some example sentences using this phrase:
"There are many fish in the lake." (在湖里有许多鱼。)
"The fishermen caught so many fish today!" (渔夫今天捉了这么多鱼！)
"There are so many different types of fish in the ocean." (海里有这么多不同种类的鱼。)