Beginning with Microsoft Outlook 2010, you can open more than one Exchange account in your Outlook profile.

Microsoft Outlook 2010 supports up to 15 accounts in your profiles. But, by default it is limited to 5 accounts.

In Outlook 2013 the default is 10 accounts and the maximum allowed is 9999 accounts.

If the administrator wants to allow more (or less) than the default number of accounts, he or she need to edit the registry or apply a group policy.

The relevant key for either Outlook 2010 or Outlook 2013 is:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Exchange
DWORD: MaxNumExchange

Outlook 2010 values: a number between 1 and 15

Outlook 2013 values: a number between 1 and 9999

In the Group policy editor, the setting is under Outlook > Account Settings> Exchange > Set maximum number of accounts per profile