Indeed you haven't understood...
Each preamp channel you have - whether that's 1 input from a 4ch unit or a single channel preamp all by itself - requires a matching line input on your interface. So a Focusrite ISA 428, for example, needs 4 line inputs.
You can only add as many preamp channels as you have available line inputs. If you're talking about the original Focusrite Saffire, you're pretty limited in that regard as you can't run the line inputs and the mic inputs at the same time, so you'll have to upgrade your interface as well.