i keep getting marked wrong when i answer this in natural english. 'mereka' is plural and 'brain' singular. one would therefore hope that each could possess their own brain. however, if that seems unlikely...
we would add more here to level this insult with emphasis on (lacking even) a singular brain: 'they don't have a brain between them'.
otherwise, 'they don't have any brains' or 'they don't have brains' or 'they have no brains'.
I am not a native English speaker and I have always wondered if you can put singular after plural like "They don't have a brain". I have read a sentence "The boys brought a hatchet with them" in my English textbook. If it is grammatically correct, then "They don't have a brain" is also correct? Or maybe the boys in the latter sentence brought one hatchet for everyone.
The difference is that logically "they" would each have a brain but it is not logical to suppose that each boy must have their own hatchet and in that case it makes perfect sense to say ".. the boys brought a hatchet with them" as they may have had just one hatchet. Better to say ".. they don't have brains."
Because, semantically, each person has a brain. "They don't have brain" sounds unnatural in English, unless you are specifically referring to whether a store (toko) or the storeroom/larder of someone's house (i.e. lemari makanan) doesn't have brain available as a food substance. But that would be highly unusual and not expected by most English-speakers anyway.
Therefore, if "they" are present (referring to people or animals), then more than one brain must also be present because brains are normally inseparable from living things. Unfortunately, I forget what the specific linguistic terminology is for this concept.