function op_register_menu_meta_box() {
add_meta_box(
    'op-menu-meta-box-id',
    esc_html__( 'Custom offers Checklist (select any 2)', 'text-domain' ),
    'op_render_menu_meta_box',
    'listing'
    );
}
add_action( 'add_meta_boxes_listing', 'op_register_menu_meta_box' );
function op_render_menu_meta_box() {
// Metabox content
$getPostsToSelect = get_posts('post_type=offers&numberposts=-1');
foreach ($getPostsToSelect as $aPostsToSelect) {
?>
<label>
    <input
      type='checkbox'
      name='yourcustom_meta[]'
      class='postsToSelect'
      value='<?php echo $aPostsToSelect->ID ?>'
     />
    <?php echo $aPostsToSelect->post_title;
    ?>
</label><br />
<?php
}}

function wpse_admin_print_scripts_edit() {
  echo "<script>var limit = 2;' .
    'jQuery('input.single-checkbox').on('change', function(evt) {' .
      'if(jQuery('input.single-checkbox:checked').length > limit) {' .
        'this.checked = false;' .
      '}' .
    '});</script>";
}
add_action( 'admin_print_scripts-post.php', 'wpse_admin_print_scripts_edit' );