$args = array(
        \'post_type\'         =>  \'custompost_type\',
        \'posts_per_page\'    =>  \'1\',
        \'meta_query\'        =>  array(
                \'relations\' =>  \'AND\', // you can use OR also as your requirement
            array(
                    \'key\'   =>  \'serial-number\',
                    \'value\' =>  \'123\'
            ),
            array(
                    \'key\'   =>  \'model-number\',
                    \'value\' =>  \'456\'
            ),
        ),
        \'post_status\'       =>  \'publish\',
    );
    $posts = get_posts($args);

if(sizeof($posts) > 0){
  var_dump($posts);
}