How do I view the EWS throttling limits imposed on me?
Office 365 will apply throttling limits to your account. The default maximum concurrent migrations allowed per account for Microsoft Office 365 is 10. This will limit the number of mailboxes that you can migrate simultaneously per admin account. Office 365 no longer provides access to the Get-ThrottlingPolicy Exchange PowerShell command used to view throttling limits imposed on an Office 365 account.
You may want to use impersonation. This allows you to set the maximum simultaneous mailboxes migrations to more than 10 (example: 100).