An Arc should normally tell a small story in itself.
So each arc also should have:
- inciting incident
- complications
- crisis
- climax
- resolution
You could, of course, just pack all of that into one 10-20k long chapter, but especially with webnovels it's often more of 10-20 chapters for one arc. Often with a bit of breathing time between those story points, some SoL chapters, or just more complications.
It's not impossible to have a small and unimportant arc in 6k words, but that would leave you with not much breathing room to actually develop the world or characters. Let's say you introduce a threat to the protagonist in the chapter, your readers won't have the time to "fear for the MC" or make their own thoughts.
Think of any good movie you enjoyed. There often are 1 or 2 short breathing passages where you as viewer can sit there and think "What will happen next?" or "How will the hero survive that?". With such short arc, you won't have that effect and it'll read more like "Hero won, Hero won, Hero won". And that often feels rushed because the complication and/or resolution is missing.
Imho - and those are just my thoughts - you should at least have a chapter break after the incident, the complications and after the crisis. Just so that your readers can actually ask themselves and wonder about your story. So I would say you should at least go for 4+ chapters an arc. I personally use more of a novel format with ~50k words/arc in a lot (~35) chapters. With that you have enough time to flesh out characters and the world in between. That isn't necessary for every arc and every story, but as I said... always give the reader time to think about your story. Or else your story will end up as fast-food story... :D