To increase the number the way you want, that would be something that was global. So if you have 20 topics per page, and showing 20 viewing per topic, that would mean 400 total users.
Now you would need to fake that count as well.
A site looking busy, and a site being busy are two different things. Populate your site with fresh, relevant and interesting content. That will get the search engines to index you, and then get users to your site when they do a search. Search engines do not index you based on a fake number of people viewing your site. People do not do a search based on the amount of people reading something, they search for keywords.