The mags are 13 rounds (+1) in the chamber). You CAN fit 15, 17, 20 or 30 round mags in it. It's the same as a regular 92 - just a shorter grip and barrel.
Do not SLAM mags that are longer than the 13 rounders, though. In fact - you shouldn't SLAM mags in any semi auto if you are using longer mags than the ones that come with the gun (like a G17 mag in a Glock 19, etc). If the mag isn't being stoped by the bottom of the gun - then repeatedly slamming a longer mag can eventually break your ejector.