It runs the gamut, and much depends on the individuals involved. As offgrid says, it takes introductory work and careful planning in the beginning, and the first hours and days are where the bulk of your problems will be. But for any who'll insist on telling you it otherwise, eg. unequivocally that it cannot be done, I have the proof right here on my place that it can. I have five (yes count 'em, five) intact male dogs living together in peace, and one female, also intact. They fight when she's in heat of course, so she gets separated for that, but otherwise, the pack cooperates and lives together just fine. But then I also own a ranch, fenced for livestock, so we're talking free roam of small acreage in the country. Not a backyard, not an apartment, and never a crate. We're all outdoor animals here, myself included. They're never cooped up alone and bored 10 hours a day locked inside, they have free roam of this place. They get plenty of exercise chasing the donkey and each other, and that is key: exercise makes a world of difference. A tired dog is a well-behaved dog. They also live in peace not only with each other but with other species eg. cattle, hogs, and a donkey. And *ahem* it also goes without saying I'm a zoo, so those intact males are treated to regular intervals of release.
So, anything can be done. Obviously my conditions are ideal, but this could probably be replicated on a smaller scale in a suburban setting if you have a way to keep them exercised. Give us specifics on your setup, and folks will chime in, advising what you can do to help integrate a pack of same genders by what they have done. It'll invariably start with gradual, controlled introductions, kept short, upbeat, and repeated often. Just like training. Here again I'm at an advantage since I built a giant 7200sqft fenced habitat in my backwoods with smaller introductory pens sharing a fence wall with the habitat. This facility serves well to integrate new dogs where they can be in sight and smell of each other, living daily sharing a fence, until a week or more goes by and they're comfortable being neighbors, unfazed at the other's presence. Next steps depend on my temperament evaluations but I usually proceed to leashed introductions before that week is up, and eventually they are allowed off-leash time together in the habitat, supervised. When those tests pass, they are left in the habitat to live a few more weeks together unsupervised, and if they can cohabitate peacefully there, they are introduced to my existing pack one at a time. When everyone gets along, a new dog graduates, and is moved up front to the cleared acres to live with me and the existing pack. In this way I have built my current pack of 6 + take on fosters/rehomes for various lengths of time.
), and others that say they'll be fine with training. I've even met same sex pairs that get along fine, and their cute togetherness was unbearable 
. I don't know who to believe though.... Give me all of your anecdotes, research theses and opinions please!
. So long as training can prevent fighting, I'll consider two dogs a possibility. If there's anything that I come across that I can't deal with, I will get zer0 dogs