This setting, combined with the Spread setting, allows you to draw multiple groups of rings at once. Copies is the number of groups to draw, and Spread is their distance from the origin, in pixels. The groups are evenly spaced along a circular path around the origin: e.g. if Copies is 5, the groups will form a pentagon, centered on the origin, with a radius of Spread.
The resulting effect is different from mirroring: the groups may interleave, and usually aren't identical. As new rings are born, each group takes a turn to receive a ring, in round-robin fashion, similar to dealing cards. The groups usually have subtle differences from each other, because in a typical patch, the ring parameters are gradually changing as the rings are dealt.
The amount of interleaving depends on how far apart the groups are spread, and the ring limit. Using Trail with multiple groups of rings and a low ring limit results in an effect somewhat like a swarm of bees, or airplanes flying in formation. If the number of copies is large, the effect will be perceived as a circular origin motion rather than distinct groups.
Note that as the number of copies increases, the distance between rings within each group also increases proportionally. This occurs because the same number of rings that would normally form a single group are now being divided among multiple groups. If this is a problem, you can compensate for it by decreasing Ring Spacing.
Also note that large Spread values can cause new rings to die immediately as soon as they're born. This is probably a bug, but you can work around it by increasing Canvas Scale, or by decreasing Spread.