Section 59C. The number of members of such reserve force shall not exceed five in cities in which the number of members of the regular force does not exceed fifteen. If the number of members of the regular force exceeds fifteen, one member may be added to the reserve force for every three of the regular force above fifteen and not above thirty; one for every five of the regular force above thirty and not above eighty; and one for every ten of the regular force above eighty.