You won't like my particular answer - as many fabrics as you want and look good to you. My favorite quilts to make have two fabrics, one large focus print and a coordinating blender.
However, I am profoundly colorblind, which in my case means I can see the basic 8 - brights, but shades baffle me. So, unless I am making a jelly roll quilt, I use at most 5 fabrics, usually only 3 or 4.