Guidelines for current draw of stepper motors vs Drivers vs Power supply

Hi!
I'm building a standard C-Beam kit and have a question about the current draw of the stepper motors.

I have 3x 4.2A NEMA 23s. It says 4.2A per phase. I also have a 24V PSU that can supply 15A.

So my questions are:
1. If it says 4.2A per phase and the motor has 2 phases (4 wires) then could it potentially draw 8.4A?
2. Will the motors draw 4.2A all the time? Or just when moving? Or just when moving and facing resistance? Or is it like...

Guidelines for current draw of stepper motors vs Drivers vs Power supply