$sql = "SELECT comment FROM comment WHERE article ='$ид_на_статия'";
$result = mysql_query($sql) or die (mysql_error());
$count = mysql_num_rows($result);
if ($count == false) {
$count = 0;
}
или
$sql = "SELECT comment FROM comment WHERE article ='$ид_на_статия'";
$result = mysql_query($sql) or die (mysql_error());
$array = mysql_fetch_array($result);
$count = count($array);