Can 1 arduino control 6 stepper motors?
A servo is not the same type of motor as a stepper. A servo is controlled by a somewhat PWM signal (somewhat, because the servos position will come from the absolute pulse length, not the duty cycle). A stepper motor needs to get the different phases driven in the correct order. This is done by a driver circuit for stepper motors and you cannot use the servo shield for this purpose without any extra circuitry.

You should use drivers explicitly meant for your type of stepper motor. Most drivers will have 2 inputs: 1 direction input (HIGH level means driving the motor in one direction, LOW level in the other) and 1 pulse pin. Everytime a pulse (pin going to HIGH, stay there for a short time and going back to low; or sometimes vice versa) is reaching this pin, the stepper will do 1 step. So to let the motor spin at a certain speed, you need to provide a specific number of pulses in a specific amount of time.

Since you want to drive all steppers individually, you need 6 drivers (one for each motor) and 12 output pins on your Arduino.