Every county superintendent having more than one hundred and fifty (150), and not more than five hundred and fifty (550) teachers, under his supervision shall have one assistant county superintendent. Every county superintendent having more than five hundred and fifty (550), but not more than one thousand and fifty (1050) teachers, under his supervision shall have two assistant county superintendents. Every county superintendent having more than one thousand and fifty (1050) teachers under his supervision shall have one additional assistant county superintendent for each additional five hundred teachers, or fraction thereof, but no county superintendent shall have more than five assistants. The assistant superintendents in office at the time this act takes effect shall continue in office until the expiration of their respective terms.