Maximum concurrent migrations is the total number of connections to the source server that are available to migrate items ...

Maximum concurrent migrations is the total number of connections to the source server that are available to migrate items in mailboxes on the Gmail or IMAP server to %BRAND_OFFICE_365_SHORT% mailboxes during initial and incremental synchronization. If the value is set to 20, which is the default value, you can migrate up to 20 mailboxes at the same time.