Insertar el formulario HTML $_POST Array en MySQL usando Implode

prueba esto

$db_num = $_POST['db_num'];
$db_amnt = $_POST['db_amnt'];

$items = array_combine($db_num,$db_amnt);

$pairs = array();

foreach($items as $key=>$value){
  $pairs[] = '('.intval($key).','.intval($value).')';
}

mysql_query('INSERT INTO ' .$tablename_db. '(db_num, db_amnt) VALUES '.implode(',',$pairs));