Assume you are doing migration to Exchange Server 2010 and you want to move 50 Mailbox at a time rather than the default number.
Check the reference Tech Net article:
Throttling the Mailbox Replication Service

To increase the number of moves you will have to make the changes in MSExchangeMailboxReplication.exe.config file.
A) Go to all Exchange 2010 CAS Servers, open the below file in notepad or any file editor:
C:\Program Files\Microsoft\Exchange Server\V14\Bin\MSExchangeMailboxReplication.exe.config

B) Change the below values:

1. MaxActiveMovesPerSourceMDB = “50″
2. MaxActiveMovesPerTargetMDB = “50″
3. MaxActiveMovesPerTargetServer = “50″

C) Save the file and restart the “Microsoft Exchange Replication Service".

Cheers,