Certified GKH Gardening Expert
You need to ask yourself if this will be a shade or sunny planter?
Make sure you use a good quality potting mix and that the container has drainage holes.
Then visit your local garden center and choose flowers or foliage that you like. You can select a mix of annuals (bloom all summer) and perennials (come back each year but often bloom for only 2 or 3 weeks) or make it your spectacular annual planter.
You will know how many plants by the spacing needed between plants which is specified on the plant label.