what will happen at output current ? when two zener diode is in series?
The output current, if there is no load, will always be zero.

The circuit is not complete, you need some kind of power supply on the left side, also some current limiting element like a resistor. And you also need to say what will be connected on the right side.

Tipically you will have a "constant" voltage on the output, and depending on the load you connect, more or less current will flow. In your particular case, you have two 5.1V zeners, so the output voltage will be around 10.2V. If you connect a 1Kohm resistor, for example, you will have a current output of 10.2V/1Kohm = 10.2mA. But this was just an example, you need to complete the circuit.