Well I try and buy for a quilt/project in mind. But, there are certain fabics that call out to me so my quilt stash has to be very organized. Hubby bought me stackable units that fit into my closet in the sewing room. They are white cubes from Lowes I think called closet maid. There are fabric drawers that fit each square the keeps them dust free. (I live in the country)
But, I have good reason to stash away fabrics. We get snowstorms in the winter that I would never drive in. So on those days I get to sew, there it is! Everything I need.
To answer your question about how much to buy. If it is a blender that is a color I use all the time,I buy a large quantity. If it is a print that I think will go into maybe one project, one or two yards. (Depending on the project I have in mind.) If it is for something specific I buy the amount given for it plus enough to re-cut for mistakes. (a quarter yard or so extra)