It may seem rather excessive to have four "stunt doubles" for both Jack and Heath, and why one for Randy at all?
Keep in mind that many scenes are done for hours at a time, and not all the actors stay on the set during the entire shoot. If you've listened to the excellent interview with the actress playing Jack's mom, it tells how the "stand-in" would be there to stand where Ennis would stand while they shot close-ups of Mom's lines and reactions. Thus, you may see the back or arm or hat, etc, of a "stunt double," or, more precisely, "stand-in" many times during a film, not just when "stunts" are occurring. Maybe when they shot the wrestling scene, Randy had already gone, so they had someone just dress in his costume while they shot over his shoulder. Or maybe Randy needed a double to do some of the riding.
Notice that one of the guys doubles for both Jake and Heath. You just need someone approximately the right size and shape if you're shooting faraway shots, or over the shoulder, etc.