they are sold in single, but they should be replaced in axles sets or pair, especially with regards to brake pads. might as well replace both side of the rotors if you are going to replace both sides of pads, since the pads come for both sides.
Every wheel requires two brake pads. So, if you need to replace the brake pads on one wheel, you need
two brake pads
. You must replace both of the pads on a wheel at the same time.