I am no sure but I would say one.
Quotw from online store:
Two 250 Watt 14K Metal Halide Lamps
Nine Blue Lunar Lights and nine White Lunar Lights (for night-time illumination)
Dimensions: 36.0" X 8.25" X 4.5" without using the legs
External electronic ballast
Notice ballast not ballasts
No wait a second in the manual:
3. Plug the 3 prong power cord into GFCI protected outlets. You may use a timer to automate the lighting cycle.
Now they are saying
plug the cord into outlets, boy I am lost.
Call the place that you are looking at purchasing from they should tell you.