It get's whatever the battery can supply after the starter hogs the current which in turn drops the available voltage. Ideally 10.5 + volts but there can be a lot of variables affecting that # both directions. I have seen them function intermittently on ~ 8V as they were mis-wired at the ballast.
Once running it should get full alternator V which should be 13.8-14.2. But once again, variables can enter due to connections, wiring grounds etc.