I *think* we've had as high as 23 before, but that was including all of our adult dogs and two litters of puppies. All the puppies were gone by 12 weeks, though, so it wasn't so bad.
We have 6 right now, and that's a pretty decent number for us. We had 5 up until Sunday, and that was the lowest number for us in a long time.
That being said, 4 of the 6 are Mini Aussies, so they hardly count. And the other two, currently, are Border Collie crosses, where as before it was one Border Collie and one GSD.
ETA: I'd like to point out that of those six in our house right now, they're each divided into "who belongs to who" between 3 owners. My sister has one dog, Morrie, who she "works" with (which is mostly throwing a ball for and doing some obedience every once in a while, but he sleeps with her and is bonded to her). My mom has two, Rhyme and Tinky, who she does agility with. And I have my permanent two, Zoey and Psyche, who both do agility, plus my foster, Crash, who I'm working on obedience and the foundations for agility with.
We've had people look at us like we're crazy because we don't just "have dogs" for the family, we have dogs who belong to each person in the house instead of all of us just kind of taking care of them. Then again, the people who think we're strange also don't have 6 dogs who NEED work.