What's the minimum amount of transistors I need to operate a stepper motor?
What's the minimum amount of transistors I need to pilote a stepper motor?

I don't know what you mean by "pilote", but if you are asking what the minimum number of transistors are to operate a stepper motor, then the answer is 0.

To make a stepper motor rotate, at a minimum you need to switch ends of some coils between open, and connected to a power supply.  This does not require transistors.  You could, for example, manually make the sequence of connections, use relays, or other mechanical means.

A interesting demonstration would be to set up brushes on a shaft to go thru one step sequence.  Each rotation of that control shaft would cause one step in the stepper motor.  It would be kindof like a "electrical gearbox".  You could add a little disclamer to the bottom: "No transistors were abused in the making of this demonstration".