Hi,
Lets simplify it this way: Bus is the connection between the physical Input (or Output) of your Audio Device, and the virtual Channel in Cubase. So you definitely need a bus to get the signal from your Input to the track, or from your track to your Audio Device Output.
But you don’t need a unique bus for every single track, of course.
If your aproach is the Addictive recording (you Record only the Guitar, then only the Bass, then only the Voice), one Bus is enough for you on the Input side. You will use the Mono In bus only. Once, you connect your Guitar, next time you will connect the Bass, and last the microphone. Still the same Input (Input 1) on your Audio Device, and still the same Bus.
I hope, I make it clear.