Increasing the number of simultaneous mailbox moves requires a change of the MSExchangeMailboxReplication.exe.config located in %ProgramFiles%\Microsoft\Exchange Server\V14\Bin. You will need to increase the highlighted sections below as you see fit. Note, this change needs to be made on all CAS servers in the organization and will need to be re-applied following any service pack updates.

<MRSConfiguration 
    MaxRetries = "60"
    MaxCleanupRetries = "5"
    MaxStallRetryPeriod = "00:15:00"
    RetryDelay = "00:00:30"
    MaxMoveHistoryLength = "2" 
    MaxActiveMovesPerSourceMDB = "5"
    MaxActiveMovesPerTargetMDB = "2"
    MaxActiveMovesPerSourceServer = "50"
    MaxActiveMovesPerTargetServer = "5"
    MaxTotalMovesPerMRS = "100"
    FullScanMoveJobsPollingPeriod = "00:10:00"
    MinimumTimeBeforePickingJobsFromSameDatabase = "00:00:04"
    ServerCountsNotOlderThan = "00:10:00"
    MRSAbandonedMoveJobDetectionTime = "01:00:00"
    BackoffIntervalForProxyConnectionLimitReached = "00:30:00"
    DataGuaranteeCheckPeriod = "00:00:10"
    DataGuaranteeTimeout = "00:30:00"
    DataGuaranteeLogRollDelay = "00:01:00"
    EnableDataGuaranteeCheck = "true"
    DisableMrsProxyCompression = "false"
    DisableMrsProxyBuffering = "false"
    MinBatchSize = "100"
    MinBatchSizeKB = "256" />

After making changes, restart the Exchange Mailbox Replication service.

net stop MSExchangeMailboxReplication
net start MSExchangeMailboxReplication