$res = mysql_query("SELECT COUNT(pa)  FROM `bo_niks` WHERE `pa` = '$pa'");
$row = mysql_fetch_row($res);
$total = $row[0];

if ($total == 10){
$num = mt_rand(5,10);
} 
else if ($total == 100){
$num = mt_rand(20,30);
}
else if ($total == 500){
$num = mt_rand(40,50);
}
else $num = mt_rand(1,5);

$bd->Query("SELECT pa  FROM `bo_niks` WHERE `pa` = '$pa'");
if($bd->NumRows() > 0){
	$row_cnt = $bd->NumRows();
	printf("В выборке %d рядов.\n", $row_cnt);

    /* закрытие выборки */
    //$bd->close();
}

switch ( (int)$bd->mRows() ) {
  case 10:
    $num = mt_rand(5,10);
    break;
  case 100:
    $num = mt_rand(20,30);
    break;
  case 500:
    $num = mt_rand(40,50);
    break;
  default:
    $num = mt_rand(1,5);
}

$bd->Query("SELECT pa  FROM `bo_niks` WHERE `pa` = '$pa'");
switch ( (int)$bd->NumRows) ) {
  case 10:
    $num = mt_rand(5,10);
    break;
  case 100:
    $num = mt_rand(20,30);
    break;
  case 500:
    $num = mt_rand(40,50);
    break;
  default:
    $num = mt_rand(1,5);
}