According to the definition of Stacked:
This means that in order for a cone to be Stacked, either that cone or one of the cones underneath it must be fully nested on a mobile goal.
It was clarified in this thread that if a cone is fully nested on a mobile goal or Stacked cone, that cone doesn’t count towards the Possession limit, even if the robot is touching it. (The robot would be allowed to touch an unlimited number of cones on this stack, even if it is also touching one un-Stacked cone.) However, the cone still needs to be fully nested.
To answer your question:
If more than one cone is stacked on each other but not on a base, it is not legal for a robot to possess it. (The robot could touch the top cone, but no other cones.)