It depends on what size quilts you like to make. Let's say you are thinking of a wall hanging or lap size quilt i would buy 2 of the same precut and then couple of the different fabrics to coordinate (at least 2 yards each) for borders and sashing, then you could use all what is left to make a unique backing.
If you like big quilts then i wouldn't buy less that 3 yards of each fabric and like (4-5) ones to make the full quilt with borders and sashing whatever needed.