In Chinese numbers are also prefixed to measure words or classifier. eg; "How many brothers do you have?" No one will answer "m" but "m'go". So, I don't believe in colloquial Chinese that a pure number is a word. You are welcome to disagree.
By the way, it is "m" for 5 in Cantonese. Unless you were talking about Shanghainese, but then is "n". "ng" is a common starting consonant in Cantonese, like "ngao" (cow) or "ngo" (I). but it doesn't have a standard meaning.
Reasonable people can disagree, but I think it's safe to say that mh would be broadly considered a word by most. And anyway, languages are generally not defined by how they are written, but by how they are spoken. Indeed many languages have no formal written form (heck, in cantonese there are major grammatical words with no written form at all.)
> By the way, it is "m" for 5 in Cantonese.
It is ngh, low rising tone. I'm sure you can verify this online.
There is no official romanization of Cantonese. Both the negative prefix `mh` and 5 are bilabial nasal, not velar nasal. They are of different tones though. Ans also as I pointed out earlier, there are velar nasals in Cantonese. to use your own logic:
> languages are generally not defined by how they are written, but by how they are spoken
So here is my understanding: one nasal that is never used in isolation is not a word.
There is, it is called 粵拼 or Jyutping (https://www.lshk.org/jyutping). However, its use is not enforced, and the original British colonial era romanisation remains frequently used in HK.
There is also the Yale system that predates Jyutping and is still also in wide use in teaching materials and dictionaries.
Therefore, 唔 is romanised as «m4» in Jyuping and as «m̀ h» in Yale. «4» in Jyutping and the backtick over «m» indicate the low falling tone.