There is much debate about the correct usage of plural forms of Japanese words. Should you say “10 hakama” or “10 hakamas”? What about “10 futon” vs. “10 futons”? Let’s investigate and see if we can’t find an answer.
In Japanese, there is no separate form of the word “hakama” to distinguish plural from singular. Or rather, the plural form of “hakama” is “hakama”. So, you can have 1 hakama or 10 hakama.
Generally, the westernized plural form of “hakama” is “hakama” (not “hakamas”). However, as there is no official standard for incorporating japanese words into English phrases, both “hakama” and “hakamas” could be considered correct. To say “10 Hakama” implies a stress on the japanese nature of the word. The speaker effectively stops speaking English and switches to Japanese. On the other hand, to say “10 hakamas” incorporates the word into the English language.
The grammar we associate with the word will typically depend on the context within which we first learned the word. For example, words like Honda, futon, bonsai and tycoon (taikun) are so widely used in the western world that the westernized plural forms with an s at the end are most common. On the other hand, westerners who learn words like sensei, samurai and hakama usually learn them within the context of Japanese culture; in these specific cases, in the dojo. Because the words are first learned as Japanese words (as opposed to English words with a Japanese origin), they are usually spoken with the intention of evoking their Japanese character. Consequently, the Japanese grammar is applied.
My personal preference is to say “10 hakama.” In my world, the hakama belongs in the dojo. When I step into the dojo, I enter Japan. I leave behind my western culture and adopt the Japanese traditions and customs (to the extent that I am able). So, when I say “hakama”, I know that I am not speaking English. I am speaking Japanese.