
A lot depends on breed. some breeds you cannot leave dogs together as if they had a disagreement it would end in injury. With some others same sex will often enough fall out to lead to caution and separating. Yet others male dogs will not tolerate another full male. Some get on fine with their own sex and opposite sex, but more than one Male can be a problem if entire bitches are owned. Other breeds will get on fine with whatever sex combination.
There are only a few breeds where bitches will not get on, but in all breeds if bitches fall out then it is often a lifelong enmity, but it happens rarely, but with males it is more likely, but often less serious. they say males fight for breathing rights, but when bitches fight it is for breathing rights :D
You must consider what breed you have and what their original purpose was.
Any breed with fighting or baiting background is likely to be one where caution is advised when keeping multiple dogs. Even the most mild mannered will not back down once in a fight, though may rarely start or get involved in one.
Other breeds like some with strong guarding traits are naturally more dominant and again if there is a status issue, may be capable of hurting each other as not wanting to back down.
Some breeds are very sociable generally but also canine enough to compete for things that any canine would, bitches, food etc. A very few breeds just haven't it in them to compete or squabble at all. Owners are even able to keep several stud dogs with entire bitches without anyone needing to assert themselves.
Then there are of course individual differences between individual dogs, some of which are not breed typical in their behaviour characteristics.
We choose our breeds and adapt according to their needs.
I agree with Ktee, I personally would not keep dogs that I couldn't expect to get on in a group without having to pussy foot around and separate them, but in the same vein I couldn't live with both sexes having to split in season bitches up from males, so I only keep girls, though do have the odd male visitor.