It depends on what you plan to dose. If you use something like All for Reef, you only need one pump. If you do 2-part, or a 2-part Balling hybrid, you'll need more pumps. So you really need to decide that first.
I have a nano tank that I dose All for Reef with a single head Kamoer X1 Pro 2 pump. I am setting up a larger tank that will be using a 2-part Balling hybrid method (dosing 4 different things), so I just got a 5 head Kamoer X5S for that tank.