Units are fielded with the amount of models shown on the card. Some cards have things like Leader & 9 Grunts - 9, or Leader & 5 Grunts - 6. That means you can pay 9 points and get 10 models in that unit, or 6 points and get 6 models in the unit.
Every unit has a Leader model. During the game, all the models in the unit must be within the CMD stat in inches, in order to be considered In Command. If you are out of Command, certain rules now apply to the model that is out.