Add Labels to Admin Menu ( How To )

First of all, searching with wrong keyword won’t give you any right solution. It’s a hint that, you are dealing with Admin menu, and anything under a menu drop down is called a sub menu – so you should search with something like: How to add submenu to any admin menu item + WordPress?. 🙂

Anyways, you have a wrong concept like adding Unlimited sub menu under a Custom Post Type – though you can (may) do it – but that would be irrelevant if that doesn’t match with the purpose of that post type (CPT).

Now come to your question:

• Dealing with Administrative menus are documented in this Codex page.
• How to add a sub menu is documented in this Codex page

So in your case the code can be:

<?php
function register_my_custom_submenu_page()
{
 add_submenu_page(
   'edit.php?post_type=notes', //parent menu slug
   'New Page Title', //menu page title
   'New Menu Title',   //menu title
   'edit_posts', // capability
   'new_menu_slug', //menu slug/unique ID
   'submenu_callback' //callback function
 );
}

add_action('admin_menu', 'register_my_custom_submenu_page');

/* Callback Function */
/* It will be visible when you will click the sub menu */
function submenu_callback() {
   echo '<h2>HERE IS WHAT MY PAGE IS ABOUT</h2>';
   echo '<p>Here is what my page says</p>';
}
?>