var i:int;
var mc_number:int = 10;

function duplicate_mc(){
 init();
}

function init():void {
 var myTimer:Timer = new Timer(1000);
 myTimer.addEventListener(TimerEvent.TIMER, onMyTimer);
 myTimer.start();
}

function onMyTimer(event:TimerEvent):void {
 i++;
 var mc:Sprite = new Sprite();
 mc.graphics.lineStyle(2,0x000000,0.4);
 mc.graphics.beginFill(0xFFCC00,0.4);
 mc.graphics.drawCircle(0,0,10);
 mc.x = stage.stageWidth/2 + Math.random()*200-100;
 mc.y = stage.stageHeight/2 + Math.random()*200-100;
 addChild(mc);

 if (i > mc_number){
  removeChild(getChildAt(getChildIndex(mc)-mc_number));
 }
}

duplicate_mc();