I need to get the current step number while rendering a volume object
the step used for sampling the volume :
something like in light path node " transparent depth , ray depth "
In short, no. This isn't currently exposed to the user.
The Transparent Depth value is a count of each transparent object, not distance traveled within it.
There is a Ray Depth output, but it's unclear what that does.
There is a Ray Length output, but it doesn't increase inside a volume. It's measured after the sample is taken.