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.
A typical build for a newbie mage is 3 in one arcanum, 2 in another, and 1 in a third. So this is probably a good baseline for "common mage with a little experience or a little talent, but nothing special." Let's say that's the 25th percentile.
Looking at the NPCs in the M:tA book, their highest arcana are 3, 4, 3, 4, and 5, but most have several arcana at or near that level, too. They seem to favor broad skills over maxing out one or two arcana. These are supposed to be major figures in a major city, so I'd take that to mean they're near the top of the pyramid, around the 95th percentile.
So if the median is a little higher than a newly-minted mage but closer to that than to the sample NPC, a typical mage is probably 3, 2, 2, 1; 3, 3, 1; or 4, 2.
The real answer for NPCs that the players actually encounter is "a little higher than any PC." If an NPC can't do anything the PCs can't do for themselves, they're probably not going to be interested in him as an ally. If an antagonist isn't more powerful than each PC alone, she won't be much of a threat for the group. In general; a clever storyteller can make a mundane human a very dangerous opponent.
Best Answer
The ones I'd put forward are Intruders: Encounters With The Abyss and Secrets of the Ruined Temple, as they give more depth to the adversaries of the game and further flesh out possibilities for the game's two big unexplored areas: the dangerous parts of magic and the secret history of the world. (Plus, magical monsters and a detailed write-up of the Seers of the Throne.) These two books are great for giving a Storyteller tools to challenge a group of mages.
If you're interested in tinkering with the magic mechanics, add in Magical Traditions and Summoners — and perhaps World of Darkness: Second Sight. If you're interested in tinkering with all the mechanics, Mage Chronicler's Guide. (Although, frankly, World of Darkness: Mirrors is a better book.)