Which one is "right", or which one is not necessary?

Your second one is good because you have a default value. But you would not be able to set that value on object creation because you do not have an initializer for it. You could always change it later of course.

With an init it would look like this.

class Vehicle {
    var numberOfDoors: Int
    var numberOfWheels: Int

    init(withDoors doors: Int, andWheels wheels: Int)
    {
        self.numberOfDoors = doors
        self.numberOfWheels = wheels
    }
}

class Car: Vehicle
{
    var numberOfSeats: Int

    init(withDoors doors: Int, andWheels wheels: Int, andSeats seats: Int) {
        self.numberOfSeats = seats
        super.init(withDoors: doors, andWheels: wheels)
    }

    override convenience init(withDoors doors: Int, andWheels wheels: Int) {
        let seats = 4
        self.init(withDoors: doors, andWheels: wheels, andSeats: seats)
    }
}

let someCar = Car(withDoors: 4, andWheels: 4)
let twoSeatCar = Car(withDoors: 2, andWheels: 2, andSeats: 4)

Now do keep in mind here, this is not likely the way you would implement an initializer. I am doing it this way to show you how some things work.

You can see in your Car subclass, for the init, we are asking for doors wheels and seats. The seats are set directly for the new property we have in this class, and the doors and wheels are passed up to the parent initializer in the super.init call.

Now you can see another convenience init function which we will use to override the primary. We use this one if we are too lazy to set the number of seats. You will not that it will set a local constant (used only within this init function of seats as 4. We then use the doors and wheels and that static local value for seats to call the default init function. In this way we are setting a default value for seats if we are too lazy to set it in the objec setup.