Canning Racks are used to hold Canning Jars off the bottom of a Pressure Canner while you are Pressure Canning.
Use "2" Canning Racks, if your Pressure Canner is tall enough to stack a second layer of Canning Jars. Place the second Canning Rack beween the layers of Canning Jars in your Pressure Canner while Pressure Canning.
Just measure the inner diameter of your Pressure Canner. Then pick the Canning Rack that fits your Pressure Canner or maybe two Canning Racks.