A nice easy one this...
If you open your theme folder and open templates > spGroupView.php you should find a function
sp_RecentPostList('show=10&admins=0', __sp('Unread and recently updated topics'));
You can change the 10 to whatever value you wish, 15, 20 etc..
Also if you change 'admins=0' to 'admins=1' it will allow admins to see the unread posts list aswel. As it's loaded through the theme template it will be global for everybody.
As always, its recommended to make a child theme to preserve this setting, and allow theme updates to not interfere with your change.
Hope this helps.