The Oxford comma is the comma before the conjunction in a list of three or more terms, e.g. "one, two, and three". I think your sentence actually didn't need a comma anway.

Different cultures have different norms and rules. I think you're saying that, in your native language, they taught you not to use mathematical notation in prose.

They didn't teach you that about English, though. It's quite common to write "21x" for "21 times" in English and is pretty much universally understood and accepted by native speakers.

It's a more general rule about when and where to use mathematical notation. Though style rules do differ in places.

For what it's worth I've not seen "12x" in formal contexts, but if that is proper then the times symbol still shouldn't be styled as part of the maths formula imho.

The first time I heard anyone pronounce 2X as "two ex" instead of "two times" was Steve Jobs in one of his presentation.