But are such judgments, like the nature of abstractions generally, dependent on or derived from physical states? Put bluntly, if number is real but not material, then materialism is false, as it says that only material things and states are real. This is really a very long–standing debate in philosophy of mathematics: at least some mathematicians are platonist. Mathematical platonism holds that the natural numbers are real independently of the individual act of thought. (See What is Math? Smithsonian Institute Magazine.)
But it’s not a valid analogy. Remember that the subject of discussion is the nature of mind. The materialist contention is the mind can be reduced to or explained in terms of physical states of the brain. Put crudely (but then, it is a crude theory), it’s that ‘brain=mind’. So what the OP’s argument is saying, is that ‘=’ is itself not a physical state but a judgement about constitution or meaning. Furthermore, that any attempt to equate mental states or neurological data with symbolic meaning must itself rely on the very judgments which it is trying to explain. That’s where the materialist argument is circular.