The usual way to indicate a plural noun in English is to add the letter s:
Note: a frequent error is that of adding an s to a compound word that contains “man” or “men.”
Incorrect: All of the salesmens attended the meeting.
Correct: All of the salesmen attended the meeting.
Some of the oldest English nouns form their plurals by changing a letter inside the word:
one man, two men
one mouse, two mice
one goose, two geese
Some English nouns have the same spelling for singular and plural:
He shot a moose.
We saw six moose that day.
The campers cooked one fish.
They took home ten fish.
Bambi is a deer.
Twenty deer gathered in the meadow.
Note: in some old expressions you will see or hear the plural fishes:
But he said unto them, Give ye them to eat. And they said, We have no more but five loaves and two fishes; except we should go and buy meat for all this people. Luke 9,13 KJV
This plural for fish is archaic (used in the past, but no longer considered standard). A modern writer might choose to use it in order to create an old-fashioned or folksy effect.