What is a commonly accepted abbreviation with regard to stock quantity of (countable) products?
As in: "200 units" of sth, just shorter.
I've come across the form "200 pc.", but as it's coming from a non-native speaker and I haven't seen it before, I can't help having some doubts. Is this form correct?
Yes, "pc." in that context is an abbreviation of "piece" or "pieces" and it's very common, and perfectly correct. Regarding the most correct abbreviation, that's probably something that's up for debate (alternatives could include "200 x" and I've also seen "200 per") but "200 pc" is as good as any and probably more common, from my own experience at least.
200 pc (or 200 pcs) is fine if those likely to read it will understand it, but as it's not much shorter than 200 units, there doesn't seem to be much reason for using it.
