360deg / 24steps = 15 deg/step.
500cycles/sec * 1step/1cycle = 500steps/sec
500steps/sec * 15deg/step = 7500deg/sec
7500deg/sec * 1rev/360deg * 60sec/1min = 1250rev/min
The max frequency of the drive is very complicated. It has to do with inductance of the motor winding, the applied drive voltage, the drive waveform, the inertia of the rotor/mechanical load, the acceleration profile, etc., etc. Read the on-line stepper motor tutorials.