Because most miners use pools and most pools are private, independent and competing organizations, that data is not shared publicly. As miners run their own nodes and there are about 20K nodes nowadays (seen another way in this visual) we know the total number of miners is below this. With mining remaining very competitive, those who were marginal miners are getting squeezed out so the total number of miners is becoming fewer over time.
Any number thrown out there, like 5,000, for instance, would just be a wild-assed guess. For every multi-ghash/s mining farm are dozens to hundreds of gamers and others with marginal GPUs (at least as far as their mining performance) earning just a few bitcents each day. http://bitcoinstatus.rowit.co.uk/hosts.html