I'm going to make some arbitrary assumptions based on certain things before I start working on my answer just to clear things up:
I think that it's not unreasonable to say that 50% of mages who are professional, fully-trained mages, have taken at least one initiation. In addition, 70% of initiated mages have probably joined a group.
If we assume that the groups listed in Street Magic make up about 10% of the mages that have initiated that belong to groups, we have 1203 mages forming 3.5% of the population of mages who are "professional" full-time wage mages or the like. This gives an estimate of 34,000 mages, which doesn't necessarily include latent magicians or magicians who are not "dedicated" mages (i.e. people with a Magic rating of 1 or 2, burnouts, and possibly a decent chunk of adepts), so you've got a fair amount of people there that either have some powers but don't really function in society as mages, and those who have very little magical power and aren't likely to fling a spell or summon a spirit, which would be the majority of people who are capable of theoretically using magic.
This number seems a lot more realistic to me, when we consider that Lone Star, for instance, has a magical investigation group in Seattle, which is almost certainly more than six people. In addition, stuff like tempo has started to blur the line between Awakened and not, and the openness of arcana to non-mages means that the magical is potentially much more prevalent than its users.
As far as the people most likely to use magic; other than some metavariants they're mostly pretty evenly spread throughout society, being a great equalizer-the punk in the ghetto is just as likely to have mojo as a corporate head, though we can expect magic users to climb up economic classifications on account of their power compared to someone without magic.