Well, let's see. Each "coin" should have a finished size of 4.5 x 2 (5 x 2.5 unfinished). So you'll need to figure out how big, in numbers, you want it. then divide the width by 4.5 to see how many columns you need. Whatever is shy, you can make up in sashing. Then take the length and divide by 2 to see how many coins you need.

Once you know how many coins you need to make, you can figure out how many different fabrics you want to use if you plan on buying it all. You should be able to get 112 coins (5 x 2.5 rectangles) from one yard of fabric.

Hope that helps.