By default, the Maximum number of Exchange Accounts allowed per Outlook Profile is 10.
A single Exchange account is normally adequate for most of us. However, it may not be if some users have gotten themselves an Office365 Email account in addition to their Corporate Email. Previously it was possible to use the Outlook Hotmail Connector to address the problem of multiple accounts, but this is not an obvious option with Office365.
If your users are are stopped in their tracks when trying adding more Exchange accounts, there is a Group Policy setting that is worth looking at:
User Configuration => Administrative Templates => Microsoft Outlook 2010 => Account Settings => Exchange => Set maximum number of Exchange accounts per profile
Be sure to set this to a higher value if it is causing an issue.