No set number. Part of the fun of making string quilts is the randomness of choice. I found for my personality and "design esthetic," if you will, that I preferred the center strips to be uniform, and for the center trianges that come together be close in size. So I guess I tried to control the size of the end pieces. OTher than that, I tried to vary the strip widths and colors. I also kept everything cool. I don't like mixing warm and cool colors. Also remember that black will grab the most attention, so use it judiciously. Decide the skinniest and widest strips you would like to see finished, and then add 1/2". I like 1-2.25 best.