I've done mailbox moves of more than 5 from one database to another.
What Exchange will do is batch them in 5's.
Then as one mailbox is done, it will auto start another, and then continue until all are complete.
We have move a few hundred, left it, and in the morning were all done.
Last Wiki Answer Submitted: November 18, 2009 8:42 pm by Grube135 pts.
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Microsoft’s Bit on Exchange Mailbox Migration
Hope this helps!
-Schmidtw