Groups of 5-10 cabins are the most common (personally I like groups of 5 the best).
I think one on one exchange have the potential to be uncomfortable because it is much more noticeable if someone over gifts\under gifts when it is just with one other person. When you are getting multiple gifts it is easy to shrug off gifts that seem to have no thought or value (or skips giving at all) and not feel as embarrassed when someone spends a lot more than you because chances are they are the outlier in the group. JMO.