I think OldGuy hit the nail on the head - the fact that it is a fantasy setting means it won't be much like reality unless it's a very, very low-magic culture. Maybe you could start with historical numbers as a base, and then ask yourself how the monsters and magic that inhabit your world would change the dynamic. Not only will it affect population sizes and concentrations, but also cultures in general. For instance, if every noble has a pet mage slinging Transmute Rock to Mud around, how many nobles are going to waste their time building stone castles? Actually, in that case, all nobles probably would be mages - maybe they all just Rope Trick themselves to bed every night and let all the Muggles fend for themselves.