It may be safest to set up for plurality, and treat one as an instance of it.
All your tests still pass with handling one thing, but you're already set up with collections. It's trivial to extend for two or more cases (and zero). At least, with those tests, we were thinking about it early.
Going the other way, from one to many, may touch a lot more things. As a future requirement, a lot more assumptions were baked in and refactoring may be harder.
It may be safest to set up for plurality, and treat one as an instance of it.
All your tests still pass with handling one thing, but you're already set up with collections. It's trivial to extend for two or more cases (and zero). At least, with those tests, we were thinking about it early.
Going the other way, from one to many, may touch a lot more things. As a future requirement, a lot more assumptions were baked in and refactoring may be harder.