The one-gallon tanks? About 8.
Maybe 10 or even 12 if your hands are big enough in a 5-quart tank.
You can fit more hangers in a tank, but if it's too full, they're awkward to manage in the dark (as in, you put 9 hangers in the tank, and one hanger is outside the tank, so you drop it in the sink and have to find it in the dark and hope you didn't scratch the film in the process).
It's good to have extra hangers, though, so you can run multiple batches or have some drying or washing while you're loading the next set.