* 394.6: Secret Societies and Organizations: This category covers secret societies, including those with martial arts or espionage aspects.
* 796.8: Martial Arts and Self-Defense: This category covers various martial arts styles, which could include information about ninja techniques.
* 952: History of Japan: Since ninjas were historically significant in Japan, information on them would likely be found within this category.
* 001.9: Occult Sciences: Depending on the specific focus (like ninja mythology or supernatural abilities), this category might be relevant.
Therefore, the appropriate Dewey Decimal number for "ninja" depends on the specific context of the information. You'd need to consider the specific focus of the material you're looking for.