DOMXpath'ом можно
Код:
$doc = new \DOMDocument();
$doc->loadHTML(<<<'STR'
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
<td>4</td>
<td>5</td>
<td>6</td>
<td>7</td>
<td>
NOTE - 1
<br>NOTE - 2
<br>NOTE - 3
<br>NOTE - 4
<br>NOTE - 5
<br>NOTE - 6
<br>NOTE - 7
</td>
</tr>
STR
);
$xpath = new \DOMXpath($doc);
$elements = $xpath->query('//tr/td[8]/br');
var_dump($elements->length);//кол-во тегов br
