I am a fan of the controlled scrappy and the themed fabric lines. I have made one quilt that used only two fabric colors, another used only three colors. To my way of thinking, they were not "scrappy" but did look beautiful. Making a totally uncontrolled scrappy, such as grabbing whatever comes out of a bag, is just not in my comfort zone. My mind says I would hate it and feel bad for putting all the time into it. So, to answer your question, I guess I consider a scrappy quilt to have at least four or more colors.
I like the phrase that there are scrap and there are scrappy quilts!