What's a appropriate amount of cologne for a man to own?

As much or as little as he sees fit.

It is only a personal preference.

I have one bottle that last me about ten years, because I don't want to smell like a mall perfume counter most of the time.

I have four different bottles, and I rotate them as well as combine them with each other for the sake of variety while still being economical.

Etiquette has no rule for that. It's how much you wear that matters.