var prevNum:int = 1;

for (var i:int = 0; i< 10; i++) {
    var iStr = String(i);
    this["Btn" + iStr ].addEventListener(MouseEvent.CLICK , btnClick(i));
}

function btnClick(i_num:int) {
    return function (event:MouseEvent):void{
        trace(i_num);
    }
}