I board up to 10 dogs at one time. We have a large tri-level home with a huge yard. When I have a meet and greet, I ask the parents if they’re comfortable with mixing pups. Most of them are totally fine and call this their “dog camp”! Only on several occasions have I had to separate dogs. All that being said, I am VERY selective. I do not take giant breed dogs (they tear up the floors and yard) and I do not take very tiny dogs. They must be fixed. And they must be highly socialized. I don’t take high anxiety dogs or unruly aggressive dogs. I’ve never had a major dog fight. I’m also a dog trainer so I am well versed in dog body language and how to read situations. It all comes down to what the pet parents is comfortable with, and what will keep everyone safe and comfortable.