Polyhive is correct. You need to have enough supers for the hives and ideally some spare.... just in case. Don't forget to think about over crowding as well.
4 or even 5 supers as a working average would be a good figure to aim for, depending on how much forage is available to the bees.
It's no fun having to do an emergency extraction ... unless of course you've now't else to do.
Also, at the height of the season it's not much fun when you've got to make up kit, or find a supplier who can send you some to make up at short notice.