Crear cuenta
<html>
<head>
<title>Ejercicio</title>
</head>
<script type="text/javascript" src="../js/jquery-1.10.2.js"></script>
<script>
function contarFilas() {
var $num = document.getElementById('tblUsuario').getElementsByTagName('tr').length - 1;
alert($num);
}</script>
<body>
<table border="1" id="tblUsuario">
<tr>
<th>Nombre</th>
<th>Apellido</th>
</tr>
<tr>
<td>Hinata</td>
<td>Hyuga</td>
</tr>
<tr>
<td>Yuno</td>
<td>Gasai</td>
</tr>
<tr>
<td>Asuna</td>
<td>Yuuki</td>
</tr>
</table>
<input type="submit" value="Enviar" onclick="contarFilas()"/>
</body>
</html>
<script language="javascript" type="text/javascript">
function contarFilas() {
var tabla = document.getElementById("form1:page1:tbl_foto_empleado");
var numFilasTabla = tabla.getElementsByTagName("tr").length - 2; //siempre se resta 2
alert(numeroFilasTabla);
return true;
}</script>
<af:commandButton text="Contar filas" id="btn_contar_filas" onclick="return contarFilas();"/>