If you are using Fuel Cell technology, the answer is 4. The NY Times explains:
The range of the Equinox Fuel Cell vehicle is about 200 miles, according to G.M., and the distance from Corning to Washington is roughly 280 miles, which means Mr. Massa would have had to refill the tank with hydrogen somewhere along the way — but there are no hydrogen stations on the most direct route from Corning to Washington.
The solution: two Equinoxes.
Carolyn Markey, a G.M. spokeswoman, said Mr. Massa drove one Equinox from Corning to Harrisburg, Pa., where a second Equinox was waiting for him. He then drove it to Washington [snip]
I'm a big fan of new technology, but until there are hydrogen fueling stations available, this is just a stunt.
H/T Planet Gore.