My best guess, is that you want plain iron rings on a plated cylinder, as the two parts wouldn't get 'broken in' if they were both chrome or nickel plated, and you would have high oil consumption .
I went to the official BMW parts site, and they only list one number for piston rings, stated to be a kit, so I assume this is for two pistons .
BMW part number : 11 25 1 337 401 .