For most people, the answer to the question 'Did you use less than 5 vehicles for business at a time' will be 'Yes'. As a sole-proprietor using your own vehicle for business purposes, your answer would be 'yes'.
An example for when the answer would be 'no' is a business that owns 5 work trucks and has employees who use the trucks on a daily basis. This business will be restricted with whether they can use the standard mileage rate for all 5 work trucks.