How to choose the suitable battery for 6 stepper motors?
As others have already mentioned, stepper motors draw power continuously when they are "ON". The data sheet says the winding resistance is 32.6 ohms, so applying Ohm's law you can determine how much current each winding will draw from the battery:

V = I x R --&gt; I = V / R = 12 V / 32.6 ohms = 0.368 Amps


which is roughly the motor's rated current specification of 0.33A (as stated on SparkFun's website). Each bipolar stepper motor has two windings, each continuously drawing 0.368 amps; therefore, each motor continuously draws approximately 0.736 amps when it is energized.

Note that a stepper motor's shaft position is maintained only when the stepper motor is energized. If you de-energize the motor (turn it "OFF"), there is no magnetic field to maintain the shaft's current position, and the shaft free rotates.

Also, the Arduino's digital I/O pins cannot provide enough drive current to adequately energize your stepper motor's windings. Typically, the Arduino sends its low power digital "control" signals to an H-bridge circuit that provides the required voltage polarity and amperage to the stepper motor's windings.