In reply to More than nine weights?:
IMO it is a matter of range. For a certain use a light might be the proper choice for the lowest weight used (eg the other ones being regular or semibold and their italics — or when more contrast is needed semibold and extrabold etc.). For another purpose the lightest might be a semibold and one would need up to a superbold to get a range, depending on the amount of contrast and the median colour the designer wants.
Nine-weight-families are necessary/needed.