It depends. My spouse died. For every account I had to call and close for him I needed to send them a death certificate. (They were usually really nice about it, and some didn't require one.) For the life insurance (any and all policies) I needed a separate death certificate. For any accounts I needed to take his name OFF of I needed one. I wound up ordering 20 and I have about 7 left. I would get more than you need to have a couple left over for future stuff if it's needed.
So you may need a few, or hardly any, depending on how much business you have to do for the deceased.