“9mm” usually implies that the weapon is a semiautomatic (i.e., self-loader, as opposed to a revolver) pistol. That means a detachable magazine capacity of at least 10 rounds (’bullets’). Common sizes are 10, 12, 15, 17, and 19; the largest commonly available are 30. Larger capacity magazines are generally not available due to technical issues with reliability and bulk.
A weapon with a detachable magazine means that it can be reloaded in a few seconds or less - simply push the magazine release button, the magazine drops out, and slam a new one home. Some guns can then immediately begin firing again, others you have to actuate the slide (pull the top half of the gun back and then release it) to load the top round. Reloading does take the use of both hands. Either way, a reload, while physically easy to do (even on a revolver) provides a few seconds in which someone could charge the gunman and take him down.
Goood description sir. My two glocks hold ten and seventeen, respectively. Glock 26 and 17.