This is how common I believe magic is in Shadow World: If you use the RMSS/FRP rules, and you play an urban-cultured human you get 2 spell levels (basically a spell list up to level 2), just for growing up in a city. For me, that says everything about magic in the setting. It is a HIGH-fantasy setting, magic is much more common-place than the generic fantasy setting. (imo)
In the shadow world players guide on page 15 it points out that spell users aren't all that uncommon, that many beings have the potential to be spell users, but it's rarer for them to have the training needed to really use magic.
***Major Loremaster Legacy Spoiler***
That's kind of touched on in Loremaster Legacy. Kalen has potential, but no real training, making his attempts to use magic often a bit clumsy. I suspect in book two that's going to be different, because he is getting trained by pros at the end of book 1.
I don't read the statement in the players guide to mean that magic is common; that's just me though. As a GM if your plots need trained magic users to be common then they can be common. I personally have zero objection to a high magic Middle Earth game, but a lot of people would disagree with me. It really boils down to what game do you want to run, and what your players want to play in.