I believe a true scrap quilt has many, many differing prints, whether they are in the same color family or cover the whole color wheel. To me a quilt with less than, say, 30 or more prints isn't really a scrap quilt, but rather a quilt with multiple fabrics/prints.
I have some vague memory of reading that Roberta Horton (or somebody well-known like that) wrote that a true scrap quilt has a minimum of 75 differing prints/colors/fabrics, or something like that.
Jan in VA