My daughter LOVES dogs - had 4 at the same time for a few years! She now dog-sits at others' homes to get her dog fix since she can't have her own. It is a win-win.
Before I moved to Auckland 25 years ago, my sister and I lived about half an hour apart. We had a reciprocal arrangement that each of us would look after each others dogs, which was great as the dogs were also best friends.
I also had friends, and friends of friends, with dogs, and provided my dogs liked theirs (they usually did), and I wasn't planning on going away myself I would look after their dogs as well, so I often had a house full of dogs. I didn't charge for looking after them other than the food. I usually had them bring in their own dogs bed so it had a familiar bed. The longest they stayed was about a month, usually one or two weeks at a time.
I had a few rules - all dogs had to eat the same. My dogs eat a vet supplied raw diet, so I fed their dogs that too, and was reimbursed. Of course if theirs needed medication or something specific as well etc that was fine, but feeding one dog cheap biscuits next to another eating raw meat - that just ends in growling, or a dejected dog. They know the difference. Even with Labradors. I used to look after our lawyers dog, he was such a cheapskate, he would want me to feed her low grade biscuits, so he had to pay me up front for decent food. At the end of the day he was getting free dog minding. She was a lovely dog though, and I used to like having her.
They had to behave well on a lead. All the dogs I looked after were large, retrievers, Rottweilers, Shepherds etc. If walking 2 or 3 dogs I had to be able to walk them under control. I once took in a dog that hadn't been leash trained, and it nearly ripped my arm out of its socket. None of our friends had small dogs and I am 5'1".
They had to be washed and flea/worm treated. All my dogs are house dogs, I learnt that lesson the hard way, and had to flea bomb the house.
As I also had cats, I couldnt take cat-chasing dogs. My cats need to feel safe in their home.
Thinking back there were about 9 dogs I used to look after regularly, and 3 that I used to walk regularly. The only down side was the extra poop scooping to keep the lawn clear. The upside is that we never had any trouble with possums (a terrible pest in NZ) eating the fruit trees.
I have moved back up north now, but my sister no longer has a dog, and I dont know if I will take in extras - maybe?