I have successfully controlled my stepper motor with a L293D and Arduino. Now I want to add more motors (up to 3) each with it's own L293D driver. I'm using a single separate power supply for the motors. Will I need a regulator to stabilize the voltage and if yes why? The motors won't rotate/step at the same time. The motors are low powered 5V - 9V.
Without seeing the entire system, it is impossible to answer your question definitively. However, it seems UNLIKELY that you need a regulator just because of the additional stepper motors. Since they are stepper motors, they aren't very dependent on voltage anyway. And if only one will be operating at a time, then there seems like little or no benefit. Of course if there are OTHER parts of the circuit that need stable voltage, that is a different question (or at least a question broader than the one you asked.)