It's supposed to be 20 Japanese hunters (out of 21)... no?
Spoilers!
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Okay. Japan has at this point in the book 20+ S-rank. During the raid on the island, which hasn't happened yet in the manga, half of them got killed by Beru while they were trying to bug out (he he, bug out). I don't know why they are saying 10 in the manga especially since that was such a point of pride for the Japanese in the light novel. Mistranslation? The world may never know.
That crown prince reminds me of Griffith... after he became evil