As to all parcels in separate ownership of 20 feet or more in width as of July 1, 1960, no more than three motor vehicles, plus one motor vehicle for every full 10 feet in excess of 20 feet in width, may be placed thereon at one time; that is, if said parcel is 30 feet in width, or up to 39 feet, no more than four motor vehicles are permitted; if said parcel is 40 feet in width, or up to 49 feet, no more than five motor vehicles are permitted, etc.