Some hitch mount bike racks such as the Thule T2 Pro XT or Yakima HoldUp Evo have the ability to expand the number of bikes they can carry by purchasing an add-on. These add-ons allow you to turn your 2 bike hitch rack into a 3 or 4 bike hitch rack allowing you to carry more bikes when you need to but remove the add-on when you have less bikes to haul. Keep in mind that most hitch rack add-ons are only compatible with 2 inch hitch receivers.