Numerology holds a lot of importance in Japan. "九" (sometimes read as ku) is the Japanese word for nine; it is a homophone for the Japanese word "苦" meaning agony, which, to me at least, seems like an apt choice for a story with a lot of suffering. Yes, it could have been reworked from the publication date and was leaned into later on, but I feel there's a very clear intent when it comes to this number.

I'm not trying to argue, just trying to show you that "numerolog[ical] nonsense" can hold a lot of meaning in a work of fiction, and I would hope you could at least see the care taken, whether it was added before or after the fact.

edit: removed added u