SpawnManager.cs

using UnityEngine;

public class Spawn_Manager : MonoBehaviour {

    public Transform[] spawnPoints;
    public float invokeTime;
    public GameObject spawnObjects;
    public int maxInvoke;
    private int invokeCount;

    void Start() {
        invokeCount = 0;
        InvokeRepeating("onSpawn", 3.0F, invokeTime);

    }

    void Update() {

        if (invokeCount == maxInvoke) {
            CancelInvoke("onSpawn");
        }

    }

    void onSpawn() {

        for (int i = 0; i < spawnPoints.Length; i++) {

            //This quaternion corresponds to "no rotation" the object is perfectly aligned with the world or parent axes.
            //Side note : if you want to load a prefab from your game folder it has to be in Resources/foldername
            //Resources.Load(foldername/prefabname)
            //optional : as GameObject (cast it)
            Instantiate(spawnObjects, spawnPoints[i].position, Quaternion.identity);

        }
        invokeCount++;
    }

}