No one outside Apple (and maybe inside too) knows for certain. They don't share information like that in advance.

The 12" MacBook is popular in Asia where that screen format is allegedly more popular.

I can say that having traveled for months with a 12" MacBook it was a great little machine for what I wanted (collecting photos, checking mail and the web, and writing lots of code in Emacs). I plan to get another and make my 15" MBP behemoth a desktop.

Potentially all of them within a few years depending on how aggressively they migrate to their own CPUs.