It varies entirely by bulb. I've seen 5, 6, 7, 10, and 12.
Personally, I had 6.7v bulbs, and 5v made them look way too dim and overall crappy. I went to a local auto store chain, and picked up 4 RV brake light bulbs that had matching bases, and required 12 volts, drew 12v off the hdd cables as mentioned. They look GREAT!