While I understand that there is a wide disparity of precision when it comes to specifying what constitutes "sensory neurons/receptors", I'm trying to find an authoritative source to provide at least a rough order of magnitude estimate on the number of receptors in the human peripheral nervous system (including skin, tongue, eyes, ears; but also mechanoreceptors and proprioceptors as well). The only sources I have found so far (here and here) only provide hints at the answer. I have found estimates as high as 15 billion, and as low as 3,000,000, but none of these guesses cite authoritative sources.
As I'm sure you can tell, I am not a student of anatomy (my interests lie in cognitive science and artificial neural networks), so forgive me if my question is too imprecise. But to put it simplistically: what I want to know is, roughly how many externally-produced sensory signals (channels) does the human brain have the capacity to process? Again, I am not looking for a precise answer, but I'm hoping to at least reduce the uncertainty to less than four orders of magnitude.
CONTEXT: My purpose in this investigation is to try to quantify the magnitude of the computational capability of the brain with regard to processing external signals. If one were to imagine an "artificial nervous system" (as distinct from an artificial neural network), how many input sensors would it be able to simultaneously process if it were on par with a human brain?
UPDATE: I recognize from the comments the inherent ambiguity of my question, so let me attempt to clarify. Assuming that the number of incoming signals to the brain at any one time is limited by the number of neural connections between the brain stem and the spinal column*, can someone provide an indication of what that number might be.
* (if that assumption is overly simplistic, feel free to enlighten me)