@milk
Here is a simple example for you, as a plugin. Put the toggle switch in “Settings>Board Settings”, once active it will remove the Second set of parentheses in the column header referring to the total task count in all swimlanes, if 1 swimlane is being used.
The added condition in the template is $swimlane['nb_swimlanes'] > 1, if there were more things you wanted to hide, you would do the same.
Hope that helps.