Normally all the steppers run on the same power supply. Figure on supplying at least enough current for 2 motors running at full power, according to the nameplate amperage rating, plus a safety factor. So if the motors draw 3 amps each, supplying 8 amps should be safe. .It doesn't hurt to supply more current, it's just more expensive. The optimum voltage can be calculated according to the motor inductance, using the formula <square root of inductance times 32>. Make sure this isn't too much voltage for your drivers, though.