Bonjour, pour des raisons de performance il vaut mieux effectuer un COUNT(*).
Essaie ceci :

<?php

$res = mysql_query("SELECT COUNT(*) AS total FROM matable WHERE Adhesion != ''");
$row = mysql_fetch_array($res );
$num = $row['total'];

echo '<b><font face="Verdana, Arial, Helvetica, sans-serif" size="5" color="#333333">'. $num. '</font></b>';

?>

Je pense qu'il s'agit de la meilleur façon de procéder.

<?php

$res = mysql_query("SELECT COUNT(*) AS total FROM matable WHERE Adhesion != ''");
$row = mysql_fetch_array($rew);
$num = $row['total'];

echo '<b><font face="Verdana, Arial, Helvetica, sans-serif" size="5" color="#333333">', $num, '</font></b>';

?>

                  <?php 
    $table = "matable";  
    $nb = mysql_num_rows(mysql_query("SELECT Adhesion FROM $table WHERE Adhesion<>''"));
	echo '<b><font face="Verdana, Arial, Helvetica, sans-serif" size="5" color="#333333">'.$nb.'</font></b>'; 
?>

<?php 
    $table = "matable";  
    $nb = mysql_num_rows(mysql_query("SELECT count(Adhesion) FROM $table WHERE Adhesion<>'' AND Adhesion NOT NULL"));
	echo '<b><font face="Verdana, Arial, Helvetica, sans-serif" size="5" color="#333333">'.$nb.'</font></b>'; 
?>

SELECT count(Adhesion) FROM $table WHERE Adhesion<>'' AND Adhesion NOT NULL

                  <?php 
    $table = "matable";  
    $nb = mysql_num_rows(mysql_query("SELECT Adhesion FROM $table"));
	echo '<b><font face="Verdana, Arial, Helvetica, sans-serif" size="5" color="#333333">'.$nb.'</font></b>'; 
?>