Maximum power which can be asked from a PC?
How can I find the maximum power


It's often written on a label on the side of the internal PSU. Since PSUs vary considerably, you'd have to check your specific PSU. You'd have to deduct the power being used by the PC itself whilst your device is attached. 



Note that the 5V rail of this 400W PSU can supply more power than the 5V rail of the 700W PSU.


  through a six-pins connector.


The individual connectors also have limits. The six-pin aux cable can supply 10A at 3.3V plus 5A at 5V - Ref.