One of the most difficult problems for Kant's philosophy is explaining the relationship between the noumenal and phenomenal worlds. On Kant's view as expressed in his Critique of Pure Reason, reality is structured by so-called "concepts of the understanding," or innate categories that the mind brings to make sense of raw unstructured experience. Since causality and number are among these categories, it is problematic to say that there are "many" noumena that individually "cause" us to have perceptions of phenomena. But if the noumenal is not the cause of the phenomenal, then what is the relationship?
However that might be, it can be said that on Kant's view the noumenal is radically unknowable. Whatever concept we might want to use to categorize some noumenon or noumena, it is Kant's view that that is only a way of categorizing phenomena, so that it is something of a mystery about how we might cognize, or think about, things in themselves at all.
In large part due to the theoretical baggage of the jargon, "noumenon" is used by philosophers almost exclusively to describe this concept in Kant's philosophy.