Thanks so mush that was helpful
I did the following and it worked

#this code is inside Car Model's class
#Overriding create
@api.model
def create(self, vals):
    res = super(CarModel, self).create(vals)
    for i in range(vals["stock"]):
        super(rc.RentCar, self.env['rent.car']).create({
            'model_id': res.id,
            'car_ref' : "%s_%s" %(res.ref,i+1),
            'name' : "%s #%s" %(res.name,i+1)
        })
    return res

Hi,

You can apply your logic by overriding the create and write method of the first model.

For overriding the create method, see: https://www.youtube.com/watch?v=_-fs_NBeOLI

Thanks