I suspect any "official" numbers you find will be incomplete or skewed because they do not consider homes that subscribe to cable or satelite that ALSO have secondary TV sets in their kitchens or garages that rely on free OTA. Across the Country there have to be thousands in use but never counted.
It also depends on who you talk to. Nielsen, the ratings people, place OTA only viewers at 9-10%. Both Knowledge Networks and SNL-Kagen place the number at 15 or 16% and rising (SNL-Kagen just projected a 10% drop in pay TV penetration by the end of 2015). Here is the link to the latest Nielsen numbers. ( TVB - ADS and Wired-Cable Penetration by DMA ) They show the OTA only viewership for Las Vegas to be 8.1%. Cox Cable, who is the cable company there, estimates it to be more like 20%. You would probably have to go door to door to find out what it really was.