[Supperman]

single-plural
fool-fools
馬鹿－馬鹿等（ばから）
馬鹿者－馬鹿者ども
馬鹿者－馬鹿者達

The Japanese suffixes which are equivalent to pleural-indicating "s" or "es" in English would be "ら”, "ども", or "たち（達）".

Hope this helps.