Inventory - problem with creating new item

Hello :) I have Inventory, but i need help.

Here is my code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Inventory : MonoBehaviour {
 
     public SimpleMouseRotator camera1;
     public SimpleMouseRotator camera2;
 
     public int slotsX, slotsY;
 
     public GUISkin skin;
 
     public List<Item> inventory = new List<Item>();
     public List<Item> slots = new List<Item>();
 
     public bool showInventory;
     private bool showTooltip;
     private bool draggingItem;
     private Item draggedItem;
 
     private string tooltip;
 
     private ItemDatabase database;
 
     private int prevIndex;
 
     void Start()
     {
         camera1 = GameObject.FindWithTag("Player").GetComponent<SimpleMouseRotator>();
         camera2 = GameObject.FindWithTag("MainCamera").GetComponent<SimpleMouseRotator>();
         database = GameObject.FindGameObjectWithTag("ItemDatabase").GetComponent<ItemDatabase>();
 
         for(int i = 0; i < (slotsX * slotsY); i++)
         {
             slots.Add(new Item());
             inventory.Add(new Item());
         }
         AddItem(0);
         AddItem(1);
         AddItem(2);
     }
 
     void Update()
     {
         if(Input.GetKeyDown(KeyCode.Y))
         {
             AddItem(2);
         }
         if(Input.GetKey(KeyCode.U))
         {
             AddItem(2);
         }
         if(Input.GetKeyDown(KeyCode.Tab))
         {
             showInventory = !showInventory;
         }
         if(showInventory)
         {
             camera1.enabled = false;
             camera2.enabled = false;
         }
         else
         {
             camera1.enabled = true;
             camera2.enabled = true;
         }
     }
 
     void OnGUI()
     {
         tooltip = "";
         GUI.skin = skin;
         if(showInventory)
         {
             DrawInventory();
             if(showTooltip)
                 GUI.Box(new Rect(Event.current.mousePosition.x + 15f, Event.current.mousePosition.y, 200, 200), tooltip, skin.GetStyle("Tooltip"));
         }
         if(draggingItem)
         {
             GUI.DrawTexture(new Rect(Event.current.mousePosition.x, Event.current.mousePosition.y, 50, 50), draggedItem.itemIcon);
         }
     }
 
     void DrawInventory()
     {
         Event e = Event.current;
         int i = 0;
         for(int y = 0; y < slotsY; y++)
         {
             for(int x = 0; x < slotsX; x++)
             {
                 Rect slotRect = new Rect(x * 60, y * 60, 50, 50);
                 GUI.Box(slotRect, "", skin.GetStyle("Slots"));
                 slots[i] = inventory[i];
                 int itemCount = inventory[i].itemCount;
                 if(slots[i].itemName != null)
                 {
                     GUI.DrawTexture(slotRect, slots[i].itemIcon);
                     GUI.Label(new Rect(x * 60,y * 60,50,50), itemCount.ToString());
                     if(slotRect.Contains(e.mousePosition))
                     {
                         tooltip = CreateTooltip(slots[i]);
                         showTooltip = true;
                         //przenoszenie przedmiotów - podnieś
                         if(e.button == 0 && e.type == EventType.mouseDrag && !draggingItem)
                         {
                             draggingItem = true;
                             prevIndex = i;
                             draggedItem = slots[i];
                             inventory[i] = new Item();
                         }
                         //przenoszenie przedmiotów - upuść
                         if(e.type == EventType.mouseUp && draggingItem)
                         {
                             inventory[prevIndex] = inventory[i];
                             inventory[i] = draggedItem;
                             draggingItem = false;
                             draggedItem = null;
                         }
                         //Używanie przedmiotów
                         if(e.isMouse && e.type == EventType.mouseDown && e.button == 1)
                         {
                             print("Clicked " + i);
                             if(inventory[i].itemType == Item.ItemType.Consumable)
                             {
                                 if(inventory[i].itemCount == 1)
                                 {
                                     UseConsumable(slots[i], i, true, false);
                                 }
                                 else
                                 {
                                     UseConsumable(slots[i], i, false, true);
                                 }
                             }
                         }
                     }
                 }
                 else
                 {
                     if(slotRect.Contains(e.mousePosition))
                     {
                         if(e.type == EventType.mouseUp && draggingItem)
                         {
                             inventory[i] = draggedItem;
                             draggingItem = false;
                             draggedItem = null;
                         }
                     }
                 }
                 if(tooltip == "")
                 {
                     showTooltip = false;
                 }
                 i++;
             }
         }
     }
 
     //wyświetlanie opisu przedmiotu po najechaniu na niego myszką
     string CreateTooltip(Item item)
     {
         tooltip = "<color=#ffffff>" + item.itemName + "</color>\n\n" + item.itemDesc;
         return tooltip;
     }
 
     void RemoveItem(int id)
     {
         for(int i = 0; i < inventory.Count; i++)
         {
             if(inventory[i].itemID == id)
             {
                 inventory[i] = new Item();
                 break;
             }
         }
     }
 
     void AddItem(int id)
     {
         for(int i = 0; i < inventory.Count; i++)
         {
             //Jeżeli nie istnieje, dodaj item
             if(inventory[i].itemName == null)
             {
                 for(int j = 0; j < database.items.Count; j++)
                 {
                     if(database.items[j].itemID == id)
                     {
                         inventory[i] = database.items[j];
                     }
                 }
                 break;
             }
             //Jeżeli istnieje dodaj +1 do licznika
             else
             {
                 if(inventory[i].itemID == id)
                 {
                     if(inventory[i].itemCount != 200)
                     {
                         inventory[i].itemCount += 1;
                         break;
                     }
                 }
             }
         }
     }
     /*
     void AddItemCount(int id)
     {
         for(int i = 0; i < inventory.Count; i++)
         {
             if(inventory[i].itemName != null)
             {
                 inventory[i].itemCount += 1;
                 break;
             }
         }
     }
     */
     bool InventoryContains(int id){
         foreach(Item item in inventory){
             if(item.itemID == id) return true;
         }
         return false;
     }
 
     //Używanie itemów
     private void UseConsumable(Item item, int slot, bool deleteItem, bool deleteCount)
     {
         //switch (zostawić)
         switch(item.itemID)
         {
             //case id, np. jabłko ma id 4 -> case 4: .... coś zrób
         case 2:
         {
             print("Mnom, mnom, mnom");
             break;
         }
         }
         //jeżeli wysłana informacja o usunięciu
         if(deleteItem)
             inventory[slot] = new Item();
         //jeżeli wysłana informacja o zmianie licznika
         if(deleteCount)
         {
             for(int i = 0; i < inventory.Count; i++)
             {
                 if(inventory[i].itemID == item.itemID)
                 {
                     inventory[i].itemCount -= 1;
                     break;
                 }
             }
         }
     }
 }
 

I have stackable item. Max stacks - 200. And now, i start from 1, when i hold "Y" key it add stacks to max, 200, but here is my problem when it reach max 200 stacks it create new item (that what i want) with 1 stack but when i still adding stacks it change both items to 1 and adding stacks to max, 200. You got it ?

I really need help with this ;/