The more sharpies you put in, the prettier it will be. I gave them about 7 or 8 from a big pack with colors like aqua, orange, etc. They will even personalize it with your child's name if you request it. (Not that it's a big deal, but I didn't get the pens back)
One time I did a black mat and pillow case with metalic Sharpies. They came out great!