In a 14 to16" basket I put 3-4 plants around the sides of the basket, usually 3 as I dont put one at the back if against a wall. on the top I put one in the centre and 5-7 around the outer edge. I really think the show comes from the amount of feeding and watering you do.

Hi, welcome to GoY, if you want a ball effect, I would put about2-3 inches of soil in the basket, then putplants about every 3", as Drc says not counting the back if it's going against a wall, then put another layer of soil in the basket, another layer of plants, another layer of soil, top off with plants 3" apart each way, give it a good watering, start feeding at the next watering, and you should get a beautiful ball of colour in a few weeks, Derek.