Showing the number of users on the forum front page is not so hard. The steps are:
(1) Go to the forum front page and click on the "see how NAML generates this page" link.
(2) Now we have to find a macro that should be changed in order to display the number of users on the page. A possible macro is the "forum_footer" (view_topics > view_topics_page > forum_footer). If you can't find this macro, use the search box.
(3) Once the macro is found, click on the "Override this macro" button and put a code like this:
<override_macro name="forum_footer">
<n.overridden/>
<div style="margin:1em 0">
This forum has <n.site_user_count filter="[n.registered_filter/]"/> registered users.
</div>
</override_macro>
Line 2 calls the original
forum_footer macro so that things can continue to work as before.
Lines 3-5 prints the sentence with the number of registered users. Note that I use the registered_filter (line 4) to count only registered users (i.e., skip banned users, etc.).