Count and display the number of subheadings

Question

With this:

* Cookie counter [0/2]
- [ ] Item 1
- [ ] Item 2

I can count the number of list items or TODO items.

But what if I just want to count the number of subheadings? Is it possible to get something like this:

* Cookie counter [2]
** Item 1
** Item 2

So, I wrote a function as follows:

  (defun my-no-of-headings ()
    (interactive)
    (save-excursion
      (length (org-map-entries t t 'tree))))

But when I execute it at the beginning of the heading, it does not display the number of subheadings.

What do I do to actually see the number of headings?