Answered by Ringtail

Selected Concepts is a section in the Browse pane that displays a list of concepts from your current selection.

The numbers in the Count column show how many documents contain each concept. If you select one document, Selected Concepts will show you concepts from only that document, hence all the counts will be 1. If you select more than one document (say, a cluster or thread), it will list the concepts for all of those documents. 

For background information and steps, see View the Concepts list.