When it comes to how to arrange pillows on a couch, some of it comes down to personal preference. Designers typically like to stick to odd numbers, but you don't necessarily have to. Generally, a symmetrical arrangement feel more formal, while a layered, non-centered arrangement is more casual.
You'll also want to mixing pillow sizes. Be sure that you include textures and patterns. Actually, the best formula for mixing patterns is to have one solid color, one botanical print and one graphic print.