Sometimes people need to open more than 10 mailboxes in Outlook but this is not possible due to a limit of the default Outlook settings.

With a simple registry entry you can change this.

Key: HKEY_CURRENT_USER\Software\Microsoft\Exchange
Value name: maxnumexchange
Value type: REG_DWORD

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Exchange]
"maxnumexchange"=dword:00000015

But I wouldn't change this to a too high value - 15/20 and than you should consider in having 2 or more Outlook profiles.