Multiple fire engines and/or trucks will only show up to a single vehicle accident if there are extenuating circumstances involved which could require additional personnel. An example of this would be an accident with someone trapped in a vehicle who needs extrication.
Under this circumstance we would send a fire engine, with a paramedic, and carrying hose and water supply. This is necessary as there is a potential for fire in any vehicle accident. We would also send a fire truck which carries rescue equipment and power tools. All of these operations require enough trained personnel to manipulate equipment and extricate the trapped person successfully. Under these circumstances we would also have an ambulance respond in case there is a medical emergency.