When I trained, oh so many years ago, the rules were one patient at a time. We even had racks that could only hold the tubes needed for one patient at a time. As I began working in the real world, it has been as many as 15 at a time. I know the chances of mix ups increase when you increase the amount of samples that you work on, but if you follow the rules - identifying the sample with patient demographics on worksheets or computers, and the spin, read, record, the chances are few. When the JCAHO recommendation came out, we looked at it, but realistically, no hospital system can afford that many techs in a blood bank to meet that recommendation - if you can, bless your lucky stars, cause I certainly cannot!