i have five, which to me feels "just right," especially since two are older and don't do much and the other three are 3-4 yo.
whenever i dog sit a dog, i do so in my home, and i can tell ya that 6 dogs is too many for me.
whenever one of my dogs goes to spend the night with grammy and grampy, the house seems too quiet and out of balance.
i will say that in the future, we'll probably stick to 3-4 dogs. I don't want my SO to have more than 1 dog at a time (because I'm the one that ends up feeding it, walking it, etc) and I don't think I could spread myself so thin if I had 4 of my own. 3 works right now because, as I said, one is older and she is more like a cat, not really wanting much of my attention.