How can I detect the number of enemies?

I’m making a game with waves of enemies. When an enemy dies it sends a message to an empty GameObject saying that it died to keep track of how many enemies are left. How can I determine how many enemies there are (I hoping to do some kind of semi-random spawning).

#pragma strict

var EnemyCount : ????;
var waveNumber = 1;

function Died ()
{
    EnemyCount -= 1;
	if(EnemyCount == 0)
	{
		waveNumber += 1;
		CreateEnemies();
	}
}

function CreateEnemies ()
{
	//NotDone
}

function CreateEnemies ()
{
EnemyCount = Random.Range(10,20);

    for(var i=0; i < EnemyCount; i++)
    {
        Instantiate(enemyPrefab);
    }
}

This should be able to get you started. Its up to you to decide how many enemies each wave will have. In this example I just pick a random number between 10 and 20

You were almost there- this should do it:

#pragma strict
 
var enemyCount : int;
var waveNumber : int = 1;
var enemiesPerWaveMin : int[]; //at least this many per wave (set up in inspector)
var enemiesPerWaveMax : int[]; //no more than this many per wave

 
function Died ()  {
enemyCount--;
if (enemyCount == 0) {
    waveNumber++;
    CreateEnemies();
    }
}
 
function CreateEnemies ()  { 
var howMany : int = Random.Range (enemiesPerWaveMin [waveNumber], enemiesPerWaveMax [waveNumber]);
for (var e : int = 0; e < howMany; e++)  {

    //spawn one enemy

    enemyCount++;
    }
}