Can i say their dumplings taste terrible since nanchi can be translated as terrible tasting?

It is ridiculous to expect us to guess what negative adjective to use here. Why does it have to be "terrible" as opposed to "bad" or "awful" or "lousy" or any of the endless synonyms for "terrible". At the very least, "bad-tasting" should be accepted as a literal translation.

Yes, they need to correct the translation. To just say "terrible" could refer to other qualities, not juste taste.

Why can it not be just one dumpling?

I would think so.

I also feel 'dumpling' is a terrible translation for 饺子. Why not at least accept the tranliteration 'jiaozi' as an alternative?