When one rotor is worn or warped, a common question is whether you can replace just that one. The strong recommendation is to replace rotors in axle pairs - both fronts together, or both rears together. Here is why.
Even braking side to side
Your left and right brakes should bite equally. A brand-new rotor on one side and a worn, thinner, or differently-surfaced rotor on the other can cause uneven clamping - a pull or a shimmy under braking. Matching both sides keeps the braking balanced.
Rotors wear as a pair
If one rotor is worn to its minimum thickness or warped - see warped rotor causes - the other side on that axle has seen the same miles and is usually close behind. Replacing both now avoids a second job soon and keeps the pair evenly matched.
New pads want a fresh surface
You should install fresh pads with new rotors, and new pads bed best against a matched pair of new rotor surfaces - see resurface vs replace. Mixing an old glazed rotor with new pads on the opposite side undermines the bed-in.
The takeaway
Front and rear can be done separately, but within an axle always do both sides. Replace rotors in pairs, fit fresh pads and hardware, and bed them in for even, quiet braking.