ObjectPool.cs

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

public class ObjectPool : MonoBehaviour
{
    public static ObjectPool instance;
    private List<GameObject> poolObject = new List<GameObject>();
    public int AmountPooled = 10;
    [SerializeField] private GameObject Objprefab; // Whatever object you want to pool.

    /// <summary>
    /// Awake is called when the script instance is being loaded.
    /// </summary>
    private void Awake()
    {
        if (instance == null) {
            instance=this;
        }
    }

    /// <summary>
    /// Start is called on the frame when a script is enabled just before
    /// any of the Update methods is called the first time.
    /// </summary>
    private void Start()
    {
        for (int i = 0; i < AmountPooled; i++)
        {
            GameObject obj = Instantiate(Objprefab);
            obj.SetActive(false);
            poolObject.Add(obj);
        }
    }

    public GameObject GetPooledOjbect() {
        for (int i = 0; i < poolObject.Count; i++)
        {
            if (!poolObject[i].activeInHierarchy) // if not active in hierarchy..
            {
                return poolObject[i];

            }
        }
        return null; // else return null.
    }
}