Depending on the dance, some of the Greek ones had long lines of people circling the room. The Bulgarian dances are so fast and vigorous that shorter lines, tendrils, perhaps, form. Up to three or four people might be a line, up to six or eight if they are all of equal skill at the footwork. The point is, these dances require individuals but don't require partners.
All of my folk dancing was the International sort that Art described. I didn't partake of any of the artificial-looking American Country Music line dances.
The two-step, maybe, but not the line dances.