There is not any concrete information that I know of, just as there isn't any specific information regarding the survival rate of the Joining for Grey Wardens.
We know that it is at least partly hereditary*, but the way Thedosian society has been run doesn't allow for a natural way to determine the ratio of mage:mundane births if the system were to go on uninhibited. The altus (upper class) in Tevinter breed in mage-ness, whereas the Circle system in southern Thedas was designed to breed out mage-ness -- mages are discouraged from having unions, Wynne informs that there are "ways to prevent" pregnancies, children that result are taken away (another disincentive), and so on.
So we can't really be sure because, until this point, society has hindered the selection process. Even post-DAI, I can see the ratio still being broken by the Circle (or College of Enchanters) system, even if it is a positive one, because you will have adult mages living and working together, mostly apart from society. In that case, you will have mages joining together, as adults do, and making more mages, just as they do in Tevinter.
There wouldn't be a completely accurate way of determining the natural ratio unless there was a society where mages and mundanes freely mixed together, then you could say that it was 1/100 births, or whatever it is.
* There may be some mutation factors as well. For example, achondroplasia (causes dwarfism) is hereditary, but can also result from genetic mutation.