How to spawn enemy? (Enemies)

Hi, I've been trying to write a script for my (ZombieStyle) FPS so when the game first loads, several "enemies" will be spawned at random spawn points I have set throughout my map. (I used empty GameObjects) I want a lot of zombies walking around aimless (Have already written the AI all works fine) I just want it to be completely random so while playing I don't learn where I will find them. I want it random.

Also if I start killing zombies to quickly then there wont be any left and thats boring. So how would I make it to where if there are less that however many zombies, Instantiate however many(Random.Range maybe). Just something to keep a decent amount of zombies on the street. Sorry to ask for a script, but I've been searching for the past 4 hours and trying with the API and can't seem to make anything work.

Here is what I have so far, I have no clue what I am doing so I started it off as spawning just 1 prefab. I can't figure out how to make Instantiate work, it says it requires 4 arguements or something. If you can write it to spawn alot when the game starts that would help.

PS: My game is all in C#

 using UnityEngine;
 using System.Collections;
 
 /// <summary>
 /// This is the EnemySpawn script.
 /// This script is attached to the SpawnManager(EmptyGameObject).
 /// </summary>
 
 public class EnemySpawn : MonoBehaviour {
 
     //--------------Variables Start-----------------
 
     private   GameObject[]    enemySpawnPoints;
     private   int             enemySpawnGroup       = 0;
     public    Transform        enemyAI;
     public    bool             iAmAnEnemy         = false;
 
     //--------------Variables End------------------
 
 
     void Awake()
     {
        //Call the SpawnEnemy function.
        SpawnEnemy();
     }
 
     // Use this for initialization.
     void Start () {
     }
 
     // Update is called once per frame.
     void Update () {
     }
 
     void SpawnEnemy ()
     {
        //Find all of the enemy spawn points and place a reference to them in the array
        //enemySpawnPoints.
        enemySpawnPoints = GameObject.FindGameObjectsWithTag("SpawnEnemy");
 
        //Randomly select one of those spawn points.
        GameObject randomEnemySpawn = enemySpawnPoints[Random.Range(0, enemySpawnPoints.Length)];
 
        //Instantiate the enemy at the randomly selected spawn point.
        Instantiate(enemyAI, randomEnemySpawn.transform.position,
                            randomEnemySpawn.transform.rotation, enemySpawnGroup);
     }
 }