Don't think there is a limit of home many people you can move at a time. It really depends on the length your comfoftable with.
I'm guessing you are doing this with both servers online instead of a situation of a failed server. The longest part of the move will be the replication of the state databases. New service book doesn't take that long.
So I recommend you take a look at the size of you state databases and do your move based on the size; big batch of users with small db and small batch of users with large db.
Exchange 2007/BES 5.0.2 MR2