“Check out the brain on Brad”

You’ve only got one brain, even if you’ve got a lot of brains. Uh?

It depends on whether you’re talking about a high IQ or the physical organ in the central nervous system.

• the brain is the organ, inside the skull. You’ve only got one of those.
• someone intelligent can be referred to as having brains or a lot of brains

In colloquial speech, the “brains” of an individual could be used, though. Or, shall we say, a gory description in crime writing or the famous accidental gunshot scene in the same film…

Prevalence: low. Not the kind of thing that comes up terribly often, outside medical and biology texts.
Frequency: high. Dutch uses a plural word (hersens or hersenen) in both cases and so Dutch writers tend to talk about a person’s “brains” in the plural as the organ.
Native: hmm. Not in any technical or scientific document, but see the note on colloquial speech above.