If the fabric is 54'' wide and I need to make 13 72'' x 12'' table runners how much fabric do I need to buy in yards?

With fabric 54" wide for every two yards you will get 4 of the runners. You are going to have fabric waste and will end up with enough for 16 runners but you have to buy 8 yards to get at the least your 13 runners because of the 72" length