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I just saw a trailer of 47 Ronin. My first thought was "that doesn't sound right". We have 12 Monkeys and 13 Warriors. Why 47 Ronin? Then I recalled that there is a movie titled Seven Samurai.
I checked the words ronin and samurai with Google, and found that the plural form of ronin can be either ronin or ronins. But the plural form of samurai is samurai.
Is there any guideline on the plural form(s) of loanwords in English, especially those from Japanese?