Why don’t you have iPhone 9?

Next to the release of the iPhone 8 / 8 Plus in 2017, Apple also released the iPhone X in 2017. (X, pronounced "Ten", X is the Roman Number / Letter for Ten). iPhone X, or "10", marks the 10-year anniversary of the iPhone, with the first iPhone being announced and released in 2007. That's probably the reason why Apple skipped the 9.

It could be marketing hype, that ten is two steps ahead of eight. Or it could be market failure avoidance, in that iPhone nine would sound in German as "iPhone - nein" (not!).

It is not hard for some of us to remember that Chevrolet had trouble selling the Nova (No vá = Won't go) in Spanish speaking countries. Likewise, Colgate toothpaste (similar to Spanish for "Go hang yourself.").

Only Steve Jobs knows, and he's not talking.

I’ve always thought they skipped the 9 because Microsoft made Windows 10. So, Apple made it a big thing to keep up with consumers.