I have built both ways.
Unless you have an unusual design, a single element will need to go across the back of the oven, but there will be no element across the front because that's where the door is.
After my first build, all mine have used 2 elements, sides only, to avoid creating a situation in which the heating at the back of the oven is different to that at the front. If I'm building an oven, it's because I want the most even heating I can manage.