<script src="jquery.js"></script>
<script>
$(document).ready(function() {
alert( $("#tabela tr").length );
});
</script><table id="tabela">
<tr>
<td>1</td>
</tr>
<tr>
<td>2</td>
</tr>
<tr>
<td>3</td>
</tr>
<tr>
<td>4</td>
</tr>
<tr>
<td>5</td>
</tr>
</table>