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

So how many Exchange Accounts can you add to an Outlook 2010, Outlook 2013, 2106, or Outlook 2019 profile?

In Microsoft Outlook 2013 and newer, the default is 10 accounts and the maximum allowed is 9999 accounts. Outlook 2010 supports up to 15 accounts in your profiles, but, by default it is limited to 5 accounts.

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

The relevant Group Policy key for all supported versions of Outlook is:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Exchange
DWORD: MaxNumExchange
Outlook 2010 values: a decimal value between 1 and 15
Outlook 2013 values: a decimal value between 1 and 9999
Outlook 2016 values: a decimal value between 1 and 9999
Outlook 2019 values: a decimal value between 1 and 9999

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

If you prefer not to add it as a policy, use these keys instead. Note: if you are using the consumer or Business version of Outlook 2013 and newer, you may need to use these keys instead.

HKEY_CURRENT_USER\Software\Microsoft\Exchange
DWORD: MaxNumExchange
Outlook 2010 values: a decimal value between 1 and 15
Outlook 2013 values: a decimal value between 1 and 9999
Outlook 2016 values: a decimal value between 1 and 9999
Outlook 2019 values: a decimal value between 1 and 9999

The new setting won't take affect until you restart Outlook.

Do it for me

To set the registry key to allow 15 accounts, use this registry file. This works with Outlook 2010, 2013, 2016, or 2019. (In HKCU\Software\Microsoft\*)
15 Accounts, user keys
This registry file will set a Policy key to allow up to 15 Exchange accounts; works with Outlook 2010, 2013, 2016, or 2019.
(In HKCU\Software\Policies\*)
15 accounts