Do I need to create one project per mailbox?
You only need to create one Project per Source and Destination email system. For example, if you need to migrate 10 mailboxes from the same Source email system to the same Destination email system, then you should create a single Project under which you can create 10 Mailboxes. This will make it easier to manage all mailboxes under the same Project. This also minimizes repetitive data entry.
To view the definition of a Project, including settings which apply to all mailboxes under it, click here. To see how you can import a list of mailboxes without having to enter only one at a time, click here.