Hard to say how many, and of course the really interesting number would be to replace "MacBooks" with "laptops", to make it more general. I don't know very many people who could, for instance, just get by with a chrome book (or an iPad or whatever netty-tablety-thing). They might get by with a desktop and a chrome book or laptop and a chrome book, but everyone I know (my parents/older relatives included) at least occasionally need a "full computer" for various projects.
A set of interesting questions might be:
(1) If I have an iPad, is there a compelling argument to get a chrome book?
(2) If I have a MacBook, is there are a compelling argument to get a chrome book instead of say an iPad or Nexus tablet?
(3) If I have a Windows laptop (Dell, etc), is there a compelling argument to get a chrome book instead of a Surface, iPad or Nexus?
(4) If I have nothing, is there a compelling argument to get a chrome book?
Unless I am wedded strongly to the Google ecosystem, I can't see a real reason to get a chrome book in the above scenarios, and even then, their support on other devices is strong enough not to warrant the purchase of an additional device to carry around and in the case of (4), applications and other factors (4G/LTE) might lead me to pick one of the tablets if I didn't want a full laptop/desktop.