Basically, any plant that grows as large or larger than a 5 gallon bucket needs to be planted one plant per 5 gallon bucket/container.
Plants like Lime Green Salad tomato can be planted two per 5 gallon bucket.
http://tatianastomatobase.com/wiki/Lime_Green_Salad You may need to do some trimming, but it can happen.
There are countless smaller pepper plants like Medusa and Ornamental Thai Chili that you could plant three per 5 gallon bucket.