If you want to stick with one fabric line as a guide to matching colors and coordinating colors, do so, but as you see most of us blend fabrics from multiple lines. I will join them. I seldom purchase from a single line of fabric. Occasionally I will purchase a blender if the color is beautifully odd or a color I don't have in my stash. It all depends on the pattern I am using. All of that said, I make primarily scrappy or controlled scrappy quilts.
I guess the final message is you do not NEED to be bound to a single fabric line unless you want to. Therefore the number of fabrics you choose is dependent on the pattern you choose for your quilt.