Hi John,
An organization can migrate a maximum of 1,000 Exchange 2003, Exchange 2007, or Exchange 2010 mailboxes to the cloud using a cutover Exchange migration. To migrate more than 1,000 Exchange 2003 or Exchange 2007 mailboxes, you can perform a staged Exchange migration. You can’t migrate Exchange 2010 mailboxes using a staged Exchange migration.
If your users are less than 1000, you can use the Cutover Migration.
Thanks,
Jason Jiang