Spawnerscript only spawns 5 enemy

I don't know what you mean, is it supposed to continuously spawn? If so start a CoRoutine or at the end of SpawnEnemies do something like

 float waitTime = 5.0f; // 5 seconds
 Invoke("SpawnWave", waitTime);

I looks like isSpawning will never be set to false again. In your update function, you check is isSpawning is false. And if it is false, then you run the spawn function. This will spawn your enemies. Then you immediately check if enemies==0; After your first iteration, amountEnimies should be 14 (starts at 15, then decrements by one. So amountEnimies == 0 will be false. Now the spawn function stops. However amountEnemies has to 0 before isSpawning is set bacl to false, I don't think this will ever happen. So in your update, you will only call startWave() once, isSpawning is set to true then and never reset. I think.

     var spawnPoints : Transform[]; // Array of spawn points to be used.
     var enemyPrefabs : GameObject[]; // Array of different Enemies that are used.
     var amountEnemies : float; // Total number of enemies to spawn.
     var yieldTimeMin = 2; // Minimum amount of time before spawning enemies randomly.
     var yieldTimeMax = 5; // Don't exceed this amount of time between spawning enemies randomly.
     var enemiesToSpawnAtTheSameTime : int = 1;
     var StartTime = 5;
     var WaveWait = 5;
     var WaveText : GUIText;
     var WaveCount : int;
     var amountEnemiesPlus = 5;
     var IsSpawning : boolean;
     private var wait : boolean = false;
      
     function Start()
     {
         yield WaitForSeconds(StartTime);
         SpawnWave();
         WaweCount = 1;
         amountEnemies = 15;
     }
      
     function Update()
     {
         if(IsSpawning == false)
         {
             StartWave();
         }
     }
      
     function StartWave()
     {
        yield WaitForSeconds(WaveWait);
         SpawnWave();
     }
      
     function SpawnWave()
     {
         if(wait) return;
 if(amountEnemies > 0)
         {
             IsSpawning = true;
             wait = true;
          yield WaitForSeconds(Random.Range(yieldTimeMin, yieldTimeMax));
             for(var s : int = 0; s < enemiesToSpawnAtTheSameTime; s++)
 {
                 var obj : GameObject = enemyPrefabs[Random.Range(0, enemyPrefabs.length)];
 var pos: Transform = spawnPoints[Random.Range(0, spawnPoints.length)]; 
                      Instantiate(obj, pos.position, pos.rotation);
             }
             amountEnemies --;
             wait = false;
             
         }
     //Moved this part out of your: if(amountEnemies >0)
 if(amountEnemies == 0)
         {
             WaveCount += 1;
             IsSpawning = false;
             amountEnemies += amountEnemiesPlus;
         }
 
      }