I even have my 2 round mag extenders and that gives me 20 rounds of 9mm with one in the pipe. The double stack mags give you a lot of ammo but makes the gun thick.
The other holsters I have used for my Glocks have been to bulky and printed too easily. I am particular about my Glock holsters because I rely on them to control access to the trigger.
This one is comfy and keeps close. Standing, sitting, driving and walking.
--Galco Kingtuk, IWN Holster. $56 from Amazon. Silly name. Good Holster.