You can use either ninja or ninjas as the plural. The majority I have seen its use is merely ninja but some places add the "s"
Nin·ja (nnj) KEY
NOUN:
pl. ninja or nin·jas
A member of a class of 14th-century Japanese mercenary agents who were trained in the martial arts and hired for covert operations such as assassination and sabotage.
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ETYMOLOGY:
Japanese : nin, to endure + ja, person (from Middle Chinese tia)
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