How fast can a stepper motor run?
You can accelerate as long as you can add power to the system.

To add power, you need to create a magnetic field that leads the rotor, which means you need to apply enough power to set up the field in time before the rotor reaches it, and have it collapse in time so you don't brake. The faster the rotor goes, the more the coils' inductivity works against you there.

You can increase the voltage with increasing speed, as shorter, higher pulses will not reach the current limit of the motor because of the inductivity, but there are limits when either the added power equals the power lost through friction or the load, or the voltage exceeds the isolation inside the motor.

In either case, you need to ramp up the speed in a controlled way, and look at the back EMF to determine whether you can still increase speed.