Above there is a picture of an AK-47. This is a assault rifle that army's such as the Taliban have used. The most common magazine holds 30. But you can buy ones that can hold up to 50 or ones that can go down to 20.
The gun above is an M4 carbine assault rifle. The rifle is used by the US military in such places as Afghanistan and Iraq. The gun has an a magazine that can hold about 30 if you have one in the chamber and you pull it out and put one more in than you would technically have 31.