<select name="Stok" id="Stok">
    <option value="">Tümü</option>
    <option value="1">1-5</option>
</select>

    $ek    =    "";
    if(empty($_GET["Stok"]) and intval($_GET["Stok"])>0{
        $ek    =    " and a>0 and a<".intval($_GET["Stok"]);
    }
    
    $sql    =    "SELECT COUNT(*) FROM Urunler WHERE (Field Adı) LIKE '%$Tedarikci%'".$ek;
    $say = mysql_query($sql);