The cap is 16% total. It's said cap for mages is 10% because everybody assumes (and rightly so) that you will spec into the +6% hit talent. But the whole 10% of the rest needs to be covered by gear.
You can get by with 5-6% from gear but more hit = more reliability. Getting 2+ misses in a row while trinket and AP are procced is not something you want to witness too often. Unpleasant.