How many backup lights are permitted on a vehicle, and how can they be used?

• Only one backup light, and it must be used separately from other lights.
• Two backup lights, and they must be used separately from other lights.
• Two backup lights, and they are permitted separately or in combination with other lights.
  Explanation:

  "Two backup lights are permitted separately or in combination with other lights."
• Three backup lights, and they must be used separately from other lights.