At least one and a half acre of fenced in yards in the back with 7 different fenced areas to split dogs into. We can easily handle upward of 80 dogs comfortably although the highest I've seen it go was around 70 or so. We can do 15 or so board and training dogs separate to that.
There really is no average. There are cyclical waves with holidays being peaks in business. Most of the time there are at least 10-15 dogs there at any one time. During holidays we load up.
As for staff to dog ratio it really depends on a few things. The kind of dogs you have coming in and the staff members skill level. Someone who is experienced can obviously maintain the peace with more dogs because they can prevent issues before they happen. I've seen us watch those 70 dogs with 3 people and have no incidents.
We often house dogs that other daycare's refuse to deal with because of dog aggression or behavior issues. The more of those you have the more people you would need around or you would need to play musical yards to cycle those guys in and out or their kennels and manage them that way.
The Pet Wagon in Durham NC. Dogability is the training portion of that.