2. Consider sunlight
You also want to consider the amount of sunlight your container will be getting each day. For example, if your container will be sitting under a covered front porch that does not get much sunlight you will want to look for plants that require shade. If your container is sitting by your pool and will be getting the sunlight all day you want to look for flowers that take full sun such as a Zinnia.
3. Consider water
4. Consider size of container
The size of your container will help you determine how many plants you will need. If I am working with a round container I like to use an odd number of plants to keep the arrangement looking round like the container. Listed below is the amount of plants I usually use with their respective container sizes. Note that this can vary depending on the kind of plants you get. For example, some plants fill out a lot more than others and need much more room to grow such as a Petunia. Also, keep in mind if you are buying packs of plants you will need to buy more to fill up your container. Packs are smaller plants and will not fill out nearly as fast because most are sprayed with a growth regulator called B-NINE that stunts the growth of plants. Some greenhouses (not us) will do this to regulate the size of the plant making it easier to transport and to extend the shelf life of the plants.
4" - 6" container: 1 plant
8" - 10" container: 3 plants
12" - 14" container: 5 plants
16" - 18" container: 7 - 9 plants
20" - 22" container: 11 - 13 plants
24" - 26" container: 15 - 17 plants
Again this is just a general guideline to get you started. The number of plants you put in your container is completely up to you!
Now get out your green thumb, get your hands dirty, and have some fun!