Collestion of objects

please don't create new answers as comments, use the comment instead..

as for your question, you need to set up a simple script to keep track of the number of your items collected, and display the total. Here's a simple example:

 static var itemsCollected : int; 
 var display : boolean = false;
 
 function OnMouseDown(){
 if(!display){
 itemsCollected += 1;
 display = true;
 var mesh : MeshRenderer = GetComponent(MeshRenderer);
 mesh.enabled = false;
 yield WaitForSeconds(5);
 Destroy(gameObject);
 }
 }
 
 function OnGUI(){
 if(display)
 GUI.Label(Rect(Screen.width/2 - 60,20, 120, 20), "Items Collected: " + itemsCollected);
 }

just attach this javascript to your prefab, and it should do what you ask..

EDIT: updated this slightly, as the previous version could cause unwanted issues..