<?php
// args
$args = array(
    'post_type' => 'lejlighed'
);

// get results
$the_query = new WP_Query( $args );

// The Loop
?>
<?php if( $the_query->have_posts() ): ?>

<a href="<?php echo add_query_arg( 'post_nr','6500' ); ?>"> 6500 </a>