public float spawnDelay;
    public float difficultyFactor = 1000;
    // Use this for initialization
    void Start () {
        StartCoroutine(spawnEnemies());
    }


    IEnumerator spawnEnemies()
    {
        int nbEnemies = Mathf.RoundToInt(Mathf.Sqrt(Time.time / difficultyFactor));
        for (int i = 0; i < nbEnemies; i++)
        {
            //SPAWN YOUR ENEMIE
        }
        yield return new WaitForSeconds(spawnDelay);
        StartCoroutine(spawnEnemies());
    }