What's the official stance on mage population density in Mage: The Awakening?
Also, what are the best unofficial alternatives (and where can we read about them)?
Like a lot of the details of the nWoD, I don't believe there are any hard and fast published stats on this. I've read most of the published material and don't recall ever coming across such information.
It's likely to be highly varied based on local culture, rate of awakenings (again, no solid stats on this that I know of), attrition of the local population due to departures, deaths, etc.
Really, I think these elements (or rather the lack thereof) are features of the sandbox nature of the nWoD. Unlike vampires, who need a set density of mortals to sustain them, which can be computed mathematically on known data (minimum blood consumption and mortal healing rates) it would theoretically be possible to have a city composed entirely of mages without putting strain on the local area unless they were engaged in a lot of vulgar magic, messing with the gauntlet, etc.
I think their density should be disproportionate to the number of other supernatural beings. If the game is solely mages then the number should go up quite a bit. With the added presence of Kindred/Garou/Changelings/What-Have-You's their numbers should decrease thanks to the food chain going all funhouse mirror. Nothing in WoD likes each other and for the most part not even their own kind beyond some tolerances, thus population density of mages should reflect the superhuman carnage caused by inter "species" strife.
EDIT: Decider number two is the area. If you walk into the Arrow's dojo, maybe the off individual is a "vanilla" mortal. Sitting at Starbucks you might have a Matter mage making the coffee beans or a technomancer going nuts on the wifi, but you might not expect more than that.
I don't play Mage but I found someone who worked on this kind of problem in the past.
Alternatively I could suggest two different way to proceed with this: