I think the most I've ever worked with in a group was three, also. If you have a good group of models, and a clear, articulated vision of what you want to accomplish, it goes pretty smoothly. If you don't have both the above, it's just going to be a torture session for all involved. I've been very fortunate that when I've worked with a group, everyone has been on the same page, and very comfortable working with other models. One thing I've found with a lot of models, especially male models, is that often they're aok with shooting nude alone, but add another model into the photo and they freak out and can't/won't do it.