I follow the general rule of planting bedding plants at a trowel length apart, its amazing how well they fill out once they get going!.
Hanging baskets, you can plant a little closer, for a more fuller/instant effect. For that size of basket, I would think five plants would be plenty especially if they are the trailing type. plant them like the spots on a dice, with perhaps a taller plant in the middle.
Keeping the baskets, well watered, I find is the biggest challenge!.