The number of articles that are not published and probably need approval is shown in statistics on the dashboard. You can see it each time you login.
You can highlight this text so that you would pay attention to it each
time you login. You can do that by altering your language at
Administration > Interface Languages.
Edit the language you're currently using (by default it is English)
and select the "Labels" tab.
Find the "Number of Unpublished Articles" label using the browser search function (Ctrl-F):
You can use HTML code there, for example you can make this text bold:
And click "Save&Close".
Now reload application by pressing F5 button.
Next time someone logins to the back-end, he would see the number of unpublished articles in the new style: