Ask yourself-What kind of quilts will/do I make?
Childrens crib/lap quilts-buy childrens prints- usually several
1 yard pieces. Bright solids-1 yard of an assortment of colors.
Adult lap quilts? Scrappy? The more the merrier, that is selection, buy 1/2-1 yard pieces, you will need a wide variety of fabrics, no reason to buy large amounts of the same fabric.
Bed quilts? these quilts will require more fabrics. Scrappy?
buy 1 yard pieces of whatever you like. Don't forget to buy tone-on-tone fabrics for the background fabrics.
Bed quilts-not scrappy? You will need to figure what the pattern and size depends on. Usually a separate purchase not a stash builder quilt.
Backing fabrics- buy these fabrics when shopping at sales,
depending on what size quilts you generally make you will need to know before going to the sale the yardage amounts.