Agree with the others that it depends how much you make in a year, and how dedicated to turning and working on your compost you are. I find I need about 3 bins, as quite a lot of wood chips and shavings go into ours, so they take a while to break down.
One to be filling, then ideally turn it into a second bin then leave to compost while working on filling and turning second bin into third, by which time first should be ready to use and third bin can be turned into it.