How many annual should be planted in a 16” hanging basket
It depends on what plant you have in mind, how deep the basket is and how frequently you want to water. Obviously, the more plant mass, the more watering is required. But 16" is a good size. It can take 3-5 petunias, 3 begonias or 2-3 nasturtium. Try to visualize the plants at their mature size. You don't want them competing for sunlight or get so crowded in the interior that it is hard to water. A very dense mature planting will have poorer air circulation, something to consider if these plants are prone to foliar disease like powdery mildew.