When an Outlook 2010 or 2007 user tries to open more than 30 calendars simultaneously they may receive the following error.
You cannot view more than 30 calendars at once. To view this calendar, clear the check box for one of the calendars.
Your first instinct might be to check the Exchange 2010 throttling policies, however this behaviour is controlled by a registry setting on the client computer.
Open Regedit and navigate to HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice14.0OutlookOptionsCalendar
Edit or create a DWORD entry of CalendarMaximumNumber and set the desired decimal value. The value must be between 10 and 100 and also must be a multiple of 10 (eg 40, 50, 60, and so on).
This limit is actually mentioned in the Office online documentation for Outlook 2007.
About viewing multiple calendars
You can open a second calendar in a new window and display up to 30 calendars in side-by-side view.