It depends whether you are buying breastforms to fit a bra you currently have or are going to buy a bra to fit the forms. It also depends on personal preference and the look you are going for.
I started with one pair and now have 4 pair and choose depending on the bra and clothes I am wearing.
If you are buying online you need to take into consideration whether you are a plus size person or not. You cannot go by cup size alone. You have to consider the weight of the forms. If you are plus size a D or DD may be narrow and not fit your body shape.