My guess is they are not disclosing the number because the number would be meaningless.
Let's take the scenario: Apple has not sold vast numbers of 5c. I doubt this is the case, but for the moment just let's assume it is. Is this a good thing or a bad thing? It may actually be a good thing (!) if the primary reason is that people are thinking - the 5S is a lot better for not much more cash (this would be my view). In which case, demand for the flagship 5S would be significantly higher, and probably a good thing for Apple.
Then again, that's assuming that Apple makes more money on a 5s. I suspect so, but it may not be by very much. Given a 32Gb 5C is only 100$ short of a 16Gb 5S, and out of that 100$ Apple have increased costs of: a fingerprint sensor; higher quality camera/flash; A7 and M7 processors, and far more expensive case (materials and manufacturing process). OK they save a few $ on storage memory. Still the difference may not be that great.
Only when we get combined 5C/5S numbers and the usual cost of manufacturing estimates, will we see how successful the launches have been.