Cardinality is an issue that comes up frequently in software design. How many of each thing should there be in your system? Over time I have found the maxim zero, one, or many to be a great guide in this area. It boils the problem down to two questions:
In my experience if a design limits something to three instances, as soon as you ship your product there will be a need for it to allow four. If you find yourself specifying specific cardinalities, tread carefully. Consider if such limits are really necessary. There may be more value in being flexible.