SpawnZombies.cs

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

public class SpawnZombies : MonoBehaviour
{

    public int numberOfZombies = 5;//How many zombies do we want in our game?

    private GameObject[] allWaypoints;// list to store all waypoints
    private GameObject zombiePrefab;

    //public static GameObject[] zombies;

    public static GameObject[] ZombiesLocation;

    // Use this for initialization
    void Start()
    {
        allWaypoints = GameObject.FindGameObjectsWithTag("Waypoint");
        zombiePrefab = GameObject.FindGameObjectWithTag("Zombie");
        zombiePrefab.SetActive(false);

        //for(int i=0; i< allWaypoints.Length; i++)
        //{
        //    Debug.Log(allWaypoints[i].transform.position);
        //}
    }

    // Update is called once per frame
    void Update()
    {
        //using a counter of the amount of zombies we have at the moment. We want to avoid having to do GameObject.find too many times
        int nrZombiesExist = GameObject.FindGameObjectsWithTag("Zombie").Length;
        //Debug.Log(nrZombiesExist);

        while (nrZombiesExist < numberOfZombies)
        {
            //if (allWaypoints.Length > 1)
            //{
            //instatiates a copy of zombiePrefab. I places it at the position of one of the waypoints (randomly picks an index in the waypoint array). It gives it a new rotation and adds the object that this script is attached to as parent
            Instantiate(zombiePrefab, allWaypoints[Random.Range(0, allWaypoints.Length - 1)].transform.position, new Quaternion(), transform).SetActive(true);

            //count them manually instead of calling FindGameObjectsWithTag every time
            nrZombiesExist++;

        }
    }
}



// located the spawing
    // 1, new zombie will brith on last zombies dead place.
    // 2, random by waypoints
    // 3,

    // after zombie dead, will leave a mark, to learn where is dead often, and avoid