By Default only 2 mailboxes are allowed to move concurrently
To increase the limit of the same, please modify the below file.
Default File Location
C:\Program Files\Microsoft\Exchange Server\V14\Bin\
I wants to increase the limit to 20 so I set the value to 20 you can set as per your choice with the max limit.
MaxActiveMovesPerSourceMDB = “20"
MaxActiveMovesPerTargetMDB = “20"
MaxActiveMovesPerTargetServer = “20"
Please restart Microsoft Exchange mailbox replication service once you have successfully edited the file, you will notice that your Concurrent mailbox move request has been increased from 2 to the max set by you.