One at a time BUT if you upgrade during the last week before the account charges you for the next month you can get many more at a reduced rate for that time period. My cycle ends near the beginning of a month so I get to upgrade for most of the holidays, just make sure your selections are in the queue BEFORE you upgrade or you'll be stuck with whatever you had there when you made the change. Just remember to return you number back to what you want 24-48 hours before the next billing cycle.
I have been picking up movies from other places because Netflix seems to have slowed down considerably with shipping which is annoying BUT they do still have an amazing selection so I'm letting it slide for now. On the other hand I have been loving Ugly Betty on streaming