its like the concept of workspace... 1 desktop (workspace) of 2x2 viewports, 2 desktops of 2x2 viewports etc
if you check the output of "wmctrl -d" you will understand
example i now have 1 desktop with 3x3 viewport
0 * DG: 17280x3240 VP: 0,0 WA: 0,0 5760x1080 N/A
if i change number of desktops to 2, i will have:
0 * DG: 17280x3240 VP: 0,0 WA: 0,0 5760x1080 N/A
1 * DG: 17280x3240 VP: 0,0 WA: 0,0 5760x1080 N/A
the compiz expo/wall/cube will change viewports only though, to switch desktop (workspace) you will need another workspace switcher.. but more desktops consume more resources and I find viewports on 1 desktop enough