editor script that demonstrates how to customize hierarchy window

CustomHierarchyView.cs

using UnityEngine;
using UnityEditor;
using System.Text;

[InitializeOnLoad]
public class CustomHierarchyView  {

	private static StringBuilder sb = new StringBuilder ();

	static CustomHierarchyView() {
		EditorApplication.hierarchyWindowItemOnGUI += HierarchyWindowItemOnGUI;
	}

	static void HierarchyWindowItemOnGUI (int instanceID, Rect selectionRect) {			
		GameObject gameObject = EditorUtility.InstanceIDToObject (instanceID) as GameObject;
		DisplayActiveAndPassiveCardCount (selectionRect, gameObject);
	}

	static void DisplayActiveAndPassiveCardCount (Rect selectionRect, GameObject gameObject)
	{
		Card[] cards = gameObject.GetComponentsInChildren<Card> (includeInactive: true);
		int total = cards.Length;
		if (total > 0 && gameObject.GetComponent<Card> () == null) {
			ActivePassive pair = ProcessCards (cards);
			Rect r = new Rect (selectionRect);
			r.x += r.width - 65;
			sb.Length = 0;
			sb.Append (total).Append (" (").Append (pair.active).Append (", ").Append (pair.passive).Append (")");
			GUI.Label (r, sb.ToString ());
		}
	}

	private static ActivePassive ProcessCards(Card [] cards) {
		ActivePassive pair = new ActivePassive ();
		for (int i = 0; i < cards.Length; i++) {
			Card card = cards [i];
			if (card.gameObject.activeInHierarchy) {
				pair.active++;
			} else {
				pair.passive++;
			}
		}
		return pair;
	}
}

struct ActivePassive {
	public int active;
	public int passive;
}