This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Will help to know the word 複数 as well.
The plural of sheep is sheep.
My friend said, "You don't even know the plural of this word?" and laughed at me, and I was very hurt.
In Japanese you don’t need to change things to the plural form as often as in English.