What's the standard for making plurals of Japanese words in English?


There's the movie "Seven Samurai", which would sound strange as "Seven
Samurais", but [in the news you often read
about](https://www.google.com/news/search?q=tsunamis) "hundreds of tsunamis".
Some people say they know 500 kanjis, but I've always thought 500 kanji
sounded more correct. Is there any standard way to pluralize Japanese words
that are used in English?

(Not sure if this is a good question for here, seeing as it's more about
English.)

