Untitled

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Eventos keyUp</title>
</head>

<script>
	function countLetters(box,valor,letras){
	 var conta = valor - box.length;

	 document.getElementById(letras).innerHTML =
	 "(" + conta + ")";
}

</script>

<body>
	Username:
	<div style="width: 170px;">
		<form action="">
			<input type="text" maxlength="10"
				onkeyup="countLetters(this.value,10,'letras')"
				style="float: left;">
		</form>
		<div id=letras style="float:right;">(10)</div>
	</div>
</body>
</html>