Limit an amount of prefabs instantiated in javascript for Unity 5.

So I've been trying to get triggers to work properly with javascript and I can't seem to figure out how to limit the amount of prefabs created in a scene in this context when using (other.tag == "Player").

 function OnTriggerEnter(other : Collider)
 {
     if (other.tag == "Player")
     {
         return;
     }
         Instantiate(shot, cubePrefab.position, cubePrefab.rotation)
 
 }

Again, I'd like to know how I could limit it to lets say three objects and then the player can do it again once it leaves the trigger with function OnTriggerExit(other: Collider) .

Not sure if theirs a better way to use a OnTriggerExit function without having to create another function all over again.

Thanks.