There shall be an assumed 500 building numbers per mile. Building numbers shall be assigned on the basis of number of feet of street frontage, in increments of 21 feet per odd or even number. Only one number will be assigned per occupiable building or dwelling unit and this number shall be the number given by the addition of the last increment of 21 feet. Thus, if a building has a street frontage of 60 feet and is the first building on the even side of the street, it will be assigned the number “106”.