public class Item : MonoBehaviour {
    public int ID;
    public Sprite Icon;
    public string Name;
    public string Descript;
    public int Size;
}

    public void AddItem(Item item)
    {
        if (Item.Count > 0)
        {
            for(int u = 0; u < Item.Count; u++)
            {
                if (Item[u].ID == item.ID)
                {
                    Item[u].Size += item.Size;
                }
                else
                {
                    Item.Add(item);
                }
            }
        }
        else
        {
            Item.Add(item);
        }
    }

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

public class Inventory : MonoBehaviour
{

    public List<Item> Item;

    public GameObject TestItem;
    public GameObject TestItem1;

    public GameObject SlotsPref;
    public GameObject Inv;
    private GameObject Prefzone;
  
    private void Start()
    {
        Item = new List<Item>();
        Prefzone = Inv.GetComponentInChildren<GridLayoutGroup>().gameObject;
    }
    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            AddItem(TestItem.GetComponent<Item>());
            Debug.Log("Item+");
        }

        if (Input.GetKeyDown(KeyCode.E))
        {
            AddItem(TestItem1.GetComponent<Item>());
            Debug.Log("Item1+");
        }

        if (Input.GetKeyDown(KeyCode.I))
        {
           
            Inv.SetActive(!Inv.activeInHierarchy);
        }

        if (Inv.activeInHierarchy)
        {
            foreach (Transform child in Prefzone.transform) Destroy(child.gameObject);
            InvOn();
        }
    }

    public void AddItem(Item item)
    {
        if (Item.Count > 0)
        {
            for(int u = 0; u < Item.Count; u++)
            {
                if (Item[u].ID == item.ID)
                {
                    Item[u].Size += item.Size;
                }
                else
                {
                    Item.Add(item);
                }
            }
        }
        else
        {
            Item.Add(item);
        }
    }

    void InvOn()
    {
        
        for(int i = 0; i < Item.Count; i++)
        {

            GameObject Items = Instantiate(SlotsPref, Prefzone.transform);
            Items.GetComponent<Image>().sprite = Item[i].GetComponent<Item>().Icon;
            Items.GetComponentInChildren<Text>().text = Item[i].GetComponent<Item>().Size.ToString();
        }
    }

}