How to create an enemy wave system?

Hi,

I want to create a FPS game where enemies spawn in waves... so the first wave there might be 2/3 enemies, second wave 5/6 enemies and so on, where the enemies also get harder.

I have created 3 times of enemies for now using cubes, spheres and cylinders

I have a system now where when you walk passed a certain area, enemies spawn from spawn points I have placed.

Can someone point me into the right direction on how to go about this spawn and wave system please. I have looked through numerous websites and youtube videos but Im not too sure how to do this. Im coding isnt very advanced

Here is my current code: using UnityEngine;

 using System.Collections;
 
  
 
 public class EnemySpawnController : MonoBehaviour {
 
     
 
     public static EnemySpawnController Instance;
 
     
 
     public GameObject Enemy;
 
     public GameObject EnemyTwo;
 
  
 
     public float spawnArea = 1.5f;
 
     
 
 void Awake() {
 
         Instance = this;
 
 }
 
     
 
     
 
 public void SpawnEnemies() {
 
     //Array
 
     GameObject[] enemySpawnSystem = GameObject.FindGameObjectsWithTag("EnemySpawnPoint");
 
     //loop to create enemies
 
     int i = 0;
 
     
 
     foreach(GameObject spawnPoint in enemySpawnSystem)
 
         {
 
         int noEnemies = Random.Range(1,4);
 
         for (int j= 0; j < noEnemies; j++)
 
             {
 
                 i++;
 
                 GameObject enemy;
 
                 enemy = Instantiate(Enemy, new Vector3(spawnPoint.transform.position.x + Random.Range(-spawnArea, spawnArea), 
 
                                                        spawnPoint.transform.position.y + Random.Range(-spawnArea, spawnArea),
 
                                                        spawnPoint.transform.position.z + Random.Range(-spawnArea, spawnArea)),
 
                                                        spawnPoint.transform.rotation) as GameObject;
 
                 enemy.name = "Enemy"+i;
 
                     
 
             }
 
         }
 
     }   
 
 }