I agree with 9/1. In a round hanging planter with less than 1 square foot, I can easily plant 5--add 4 for the corners, and there you are with 9.

For the tiny Johnny-Jump-Ups, I'd even try 16/1. (I based on the planting of a large flower pot.)

Some day, I'd like to have a square-foot "quilt" of pansies in 9-patch designs for a winter garden.