Hi,
According to my Collins/Cobuild Compact English Learner's Dictionary, the definition of the word 'rationale' is listed as "The rationale for a course of action or a belief is the set of reasons on which it is based."
My question is "Why is it 'set'?" Does it mean that 'rationale' should always consist of more than one reason? In very general terms, I think this is true. One simple reason would not normally be termed a 'rationale', which suggests something with at least a bit of length and complexity.
In more rigorously logical/mathematical terms, I believe a set can consist of only one thing, so to some extent that meaning is also available here.
Best wishes, Clive