I have a subcategory with one quiz, presumably because the editors perhaps expect others to contribute in the future (it is a category for Dionysus within Greek Myth; I was surprised no one else had written about him!). My sister writes quite specialized literature quizzes, and has been given a subcategory with only two quizzes.
It could also be there were previous quizzes on the topic but those quizzes were taken offline.
In Television, if there's three or more, we generally create a subcategory. However, we only go through what needs new subcategories when we are done editing quizzes, so it's not all that often (and please don't ask! We'll get to them eventually.)