Most vehicles fight as individual units and are represented as a single model. However, some vehicles like Ork Wartraks and Eldar Vypers operate in units of more than one vehicle rather like troops, usually with up to three vehicles in a unit.
Vehicle Units
Source: Core Rulebook 3E, p. 88
Some vehicles, such as Ork Wartraks and Eldar Vypers, fight in units of up to three vehicles rather than as a single model.