Depending on the style of your sofa, your overall decor scheme, and your feelings about throw pillows, you can add as few as one or as many as you can fit. A midcentury style sofa with a low back and fitted upholstery will generally look best with fewer pillows that add interest without overwhelming the style’s clean lines. An overstuffed couch with large detachable back and seat pillows might need no throw pillows at all, or one or two small pillows in a contrasting fabric to add interest. On a couch, a pair of pillows will look more traditional and formal than groupings of 3 (or 6, or 9, depending on the size of your sofa).