You can use Group Policy to control the maximum number of items displayed in the People Pane.
The default number is 100 items but you can choose a value between 1 and 100.
Note: Microsoft has not tested this with a value that is greater than 100. I've been using it with a value of 500 and it seems to be working fine.
This policy setting is controlled by the following registry information:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\Outlook\SocialConnector DWORD: MaxWDSSearchResults Data Value: an integer between 1 and 100