How to do it...

To automatically populate a panel using UI Grid Layout Groups to follow these steps:

1. Create a new folder named Prefabs. In this folder, create a new empty prefab named panel-inventory-slot.
2. From the Hierarchy panel, drag the GameObject Panel-inventory-slot into your new empty prefab named panel-inventory-slot. This prefab should now turn blue, showing it is populated.
3. In the Hierarchy panel, delete the three GameObjects Panel-inventory-slot / (1) / (2).
4. Un-child Text-title from Panel-stars. Set the Pos-X position of Panel-stars. To 130 - so that the panel is now to the right of the text Stars.

5. With the panel Panel-stars selected in the Hierarchy panel, add a grid layout group component (Add Component | Layout | Grid Layout Group). Set Cell Size to 50 x 50 and Spacing to 5 x 5. Also, set the Child Alignment to Middle Center (so our icons will have even spacing at the far left and right), as shown in the following screenshot:

6. Replace the C# script PlayerInventoryDisplay in GameObject player-SpaceGirl with the following code:

using UnityEngine; 
using System.Collections; 
using UnityEngine.UI; 

[RequireComponent(typeof(PlayerInventory))] 
public class PlayerInventoryDisplay : MonoBehaviour  { 
   const int NUM_INVENTORY_SLOTS = 5; 
   public GameObject panelSlotGrid; 
   public GameObject starSlotPrefab; 
   private PickupUI[] slots = new PickupUI[NUM_INVENTORY_SLOTS]; 

   void Awake() {
         float width = 50 + (NUM_INVENTORY_SLOTS * 50); 
         panelSlotGrid.GetComponent<RectTransform>().SetSizeWithCurrentAnchors( RectTransform.Axis.Horizontal, width ); 
 
         for(int i=0; i < NUM_INVENTORY_SLOTS; i++){ 
               GameObject starSlotGO = (GameObject) 
               Instantiate(starSlotPrefab); 
               starSlotGO.transform.SetParent(panelSlotGrid.transform); 
               starSlotGO.transform.localScale = new Vector3(1,1,1); 
               slots[i] = starSlotGO.GetComponent<PickupUI>(); 
         } 
   } 

   public void OnChangeStarTotal(int starTotal) { 
         for(int i = 0; i < NUM_INVENTORY_SLOTS; i++){ 
               PickupUI slot = slots[i]; 
               if(i < starTotal) 
                     slot.DisplayColorIcon(); 
               else 
                     slot.DisplayGreyIcon(); 
         } 
   } 
} 

7. Ensure GameObject player-girl1 is selected in the Hierarchy. Then drag from the Project panel GameObject Panel-stars into the Player Inventory Display (Script) variable Panel-slot-grid, in the Inspector.
8. With GameObject player-girl1 selected in the Hierarchy, drag from the Project panel prefab panel-inventory-slot into Player Inventory Display (Script) the variable Star Slot Prefab, in the Inspector. Steps 7 and 8 are illustrated in this screenshot:

9. Edit the script class PlayerInventoryDisplay to set the constant NUM_INVENTORY_SLOTS to have 10 or 15 slots. So some can only be seen when using the horizontal scroll bar.
10. Save the scene and play the game. As you pick up stars, you should see more of the grey stars change to yellow in the inventory display.