Each variable port can control 2x 1-10v channels. Since the P4 comes with 3 ports, you can have 6 channels of independent control. If you're looking for full OFF control, take a look at the EVG-AP-2F-HIC accessory.
This allows you to wire it to each 2x54w ballasts so the lights can be turned OFF completely.
For 6x54w, you would need 3 of these.