Yes you do
You can purchase one or more.
No just the bad one.
You can buy just one.
According to customer reviews the best brake pads and rotors are EBC brake rotors and pads. They are manufactured in Britain and can be found at any auto store including Canadian Tire in Canada.
You can purchase break rotors online at Rotorsonline.com. Putting on break rotors is a little time consuming. You will have to take the tires completely off to put the rotors back on. It will be good if you had a vehicle lift or a really good jack.
One can purchase EBC brake pads and rotors online from stores like Auto Anything, EBC Brakes Shop, RDA Brakes, EBC Brakes Direct and eBay. These brake pads can also be purchased from other sources than the ones specified.
Purchase a repair manual for that vehicle from your local auto parts store if you are not familiar with brake repair.
Brake calipers are the hydraulic devices that wrap around the rotors and pinch the brake pads into the rotors when you stop.
You can get your brake rotors replaced by visiting Autoparts Warehouse which contain various brake rotors in size and durability to fit your needs and accommodate your budget.
Front rotors on Citroen relay are always replaced in pairs.
Excessive or heavy and continuous braking may warp brake rotors. Braking hard (heating the rotors a lot) and then going through water (which quickly cools the hot rotors) may also cause the rotors to warp. If the brake pads or brake rotors are getting thin, the rotors will warp more easily.
The brake rotors only need to be changed if they are worn beyond allowable specifications and should be changed in pairs. They do not need to be changed with the pads unless they are damaged as above or warped.