Your form doesn’t contain the fields you think it does.
Use FireBug or a similar tool (even var_dump($_POST)) to see what is actually being posted.
Your num_users is a setting group, not a single field. At best the field with the value will be user_main_settings field.
The previous commenter is also correct – checking for $_POST[‘submit’] as your trigger is dangerous.
Have a look at this tutorial: http://ottopress.com/2009/wordpress-settings-api-tutorial/