There's a few things to consider here. In terms of size, well, one person can theoretically drive damn near anything. Esp. Modern/High tech vehicles.
Now, older vessels with old-style steam plants might be a different story, it may take one or more people just to keep the power plant up and running, let alone driving.
Also, figure out what'll need to be done whiel the vehicle is in use. It's handy to have devoted gunners, comms. ops and sensor ops on a battle vehicle, since that leaves the driver to do what he does best. Calling out quick instructions to the guy in the next seat is a free action; shooting, driving meneuvers, getting a sensor lock, and jamming communications are not.