How to find stall current of servo motor?
V = IR

To the first order, stall current = Applied drive voltage divided by the DC resistance of the coil.

The stall current occurs when the motor is stalled (not moving), so there is no reactance and no "back electromotive force (EMF)". It's literally only the resistance of the wiring in the servo motor that is resisting the flow of current... just like a resistor in a DC circuit.