Look at the tan block on the bottom right with the X drawn through it. Only 2 of those triangles, say the top and bottom ones, will be used in an hourglass quilt. You don't need more fabric if you only need to make 128 hourglass blocks. You only need 64 7.5" squares of each color. But you also said that you need every other square to be a plain block. Are those from the same fabrics, or from different fabrics?