3 to 4 is ideal, but we've had up to 6 at a time. Six is hectic, but with my job change, I could make it work. I enjoy the heck out of having different dogs at different stages. Enzo and Ozzy are couch potatoes. They don't really do a whole lot except play in the yard with us and each other. Ozzy runs with Steven, but only because he's the smallest and running at Stevens pace is hard for the taller dogs.
Anyway, two higher drive dogs makes things good for me. Knox has the highest drive/intensity/requirement on my time of any dog I've ever owned, worked with or fostered before in my life, so if I had another one of him, I'd be at my limit. Luckily, Indy is a chill, relaxed dog. She is able to settle in the house, go a couple of days to a week without getting a hardcore exercise in.
So I could add 1 to 2 medium energy dogs to this without rocking the boat too badly. Just no more males, because Knox is a jerk.
And we're looking for one more. I want another Dutch, but I wouldn't say no to a Mal.... lol
And I'd like the ages to be staggered a bit more, but Oz is 6, Enzo 5, Knox and Indy are both 2. But a good four years or so in between each group of two works out.