To access the SiteManager interface settings panel click the "Settings" > "System Settings" link from the top menu. You will be shown a list with all the available settings for your installation. Find the "SiteManager Interface Settings" section and click the "View/Edit Settings" button or double click the row.
From the "Lists & Pagination" section you can control how many items are shown on each page in the SiteManager.
After selecting the number of items per page, click the "Update Settings!" button. The settings will be saved but will not be applied to the site. To apply the new settings you will have to rebuild the configuration file. To do that find the green "Apply Settings" button above the form and click it.
Please note that these settings are only for the SiteManager interface and not the public website!