It depends on what size 1/2 square triangles you plan to make and what size quilt you plan to make. In the tutorial they use a layer cake - which is 10" squares, so, after making your triangle squares & trimming, squaring them up you probably have 8 1/2" squares ; 8" finished.... So, 20 squares ( 4 across X 5 down) would make it 32" x 40" and takes 20 (10") squares... In a yard of fabric (36" X 42") you can cut 12 (10") squares (3 across X 4 ) so, you need to determine the size of the quilt you want to make, their size of the squares you want to use and how many you can get out of a yard of fabric