How much motors you can drive depends on the shield.
Official Arduino Motor Shield:
According to the documentation of David Cuartielles it says:
"The IC used on the motor shield can be used to drive one stepper motor, or two DC motors controlling speed and direction, or four DC motors controlling with fixed direction."http://blushingboy.net/p/motorShieldV3/page/Background/
MotorShield from adafruit
According to the documentation of ladyada:
"# 2 connections for 5V 'hobby' servos connected to the Arduino's high-resolution dedicated timer - no jitter!
# Up to 4 bi-directional DC motors with individual 8-bit speed selection (so, about 0.5% resolution)
# Up to 2 stepper motors (unipolar or bipolar) with single coil, double coil, interleaved or micro-stepping. "http://ladyada.net/make/mshield/index.html#
Edit: "up to x motors" means that you can either drive for example 4 DC motors or 2 stepper motors.