* SPAM *

This how I recently solved this problem:

var thePrefab : GameObject;
var MaxObjectCount = 0;


function Update () {

    if(Input.GetButtonUp("Fire2")){
    var instance : GameObject = Instantiate(thePrefab, transform.position, transform.rotation);
       instance.name = "boxSpawn";
         MaxObjectCount += 1;
         checkhit();  
    }
}

function checkhit(){
    if(MaxObjectCount == 16){
       Destroy(GameObject.Find("boxSpawn"));
       MaxObjectCount -=1;
    }
}

Where the var 'MaxObjectCount' adds one each time my Prefab spawns in the Update function, and in my 'checkhit' function I set it to destroy the first one I spawned after a set limit. So I can only have 15 clones at a time.

I'm fairly new and don't know if this is the best approach, but it works. I am now working on allowing the MaxObjectCount var to speak to the prefab so I can also keep track of the info to be able to do the same when I destroy my prefab via a collision.

I hope this helped.