In the 1931 NEC the requirement was one switch (disconnect) for each set of service conductors, with an exception for single-family dwellings which were allowed to omit that switch if there were no more than 6 branch circuits (there is the first appearance of the maximum of 6 disconnects per service).
Prior to that (my 1928 and earlier editions), the references all state that each set of service conductors have "a switch", "the switch", etc., all with the singular references of "a" or "the" in reference to the singular reference "switch", not the plural reference switch"es".
Thus, only one was allowed until 1931, then for single-family with not more than 6 branch circuits was allowed to eliminate that one, leaving six.