The dealership will provide a single spring, tailored to the VIN of your vehicle and unique to the installed extras of your individual automobile.
It is often advisable to replace the rubber pad concurrently, given the little expense, while you are already engaged in the task.
As previously said, it is often advisable to replace springs in pairs; however, if the failed spring is relatively fresh and its malfunction is attributed to an incident such as traversing a pothole, and the other spring is still in satisfactory condition, you may choose to replace just the defective spring.
If the defective spring is under one year old and you possess a receipt, you may file a warranty claim, provided it was not damaged due to a pothole or similar circumstances.
Regarding aftermarket options, I recommend Bilstein or another respected brand; however, please note that they are more generic and do not serve as precise replacements for your original springs. Consequently, it would be advisable to get a pair, since they are sometimes supplied only in pairs.