The maximum the motors will draw is 2/3 the rated current of each motor, so you really don't need more than 8 amps.
If it's a 4 wire motor, then one coil. If it's an 8 wire motor, then measure it as it would be attached to the drive, either series or parallel.
A 48V supply would work, but the motors may run a bit warmer. Maximum speed is proportional to voltage, so as you decrease voltage, your potential maximum speed will decrease.
How the machine is geared and driven may have an effect on this as well. If you're not using the maximum available speed of the steppers, then you may not see any difference from a lower voltage, other than cooler motors.