6V is voltage, not current. If you want to drive any number of 6V motors, you will always use a 6V supply.
What you're interested in is the Amperage (i.e., current) drawn by the motors. Find out the current required by each motor, and multiply that by the number of motors. If, for example, your 6V motors draw 1A each, then you would need at least a 6V 2A supply.
If you are talking about the Adafruit motor shield, keep in mind that the maximum current per motor is 0.6A. There are ways to increase this if needed. They are discussed in this FAQ: viewtopic.php?f=31&t=26873