We have 3 Berners right now -- this fall we took in those 2 older girls that Doug mentioned, but as some of you may recall, Canoe bloated and we lost her. Jenny (9 yrs and going strong!) has adjusted well and we are a big happy family - mom, dad, toddler, 3 dogs and 2 cats. Sometimes I wish that we only had one dog/cat because of the expense/time/training etc. involved in having multiples. But, I can't imagine going back to that. On bad days, I can curl up with 3 dogs and just rest between their warm bodies and relax. The "pack dynamics" are fascinating to watch. And, I have "built-in" distractions for my dogs to work with. I can work one dog on a Stay while Heeling with the other one. And I've found that our very shy, super-sensitive rescue, Shadow, benefits from being part of a larger canine group. When we had all 4 I could use "pack dynamics" to encourage him to venture out into parts of our property that were scary to him. He would go along, I guess because he figured that if the other dogs were okay with it, it was probably safe.
Others have already listed pros and cons and helpful tips for adding dogs to the group. I would say that having added both rescues and a pup to the group that it is of the utmost importance to take temperament into consideration. All dogs go through adjustments when the group makeup changes, but these can be helped by knowing your dogs and choosing a suitable newcomer. Catherine Young Madison WI Shadow, Mickey & Jenny