Two is probably a good idea if you have more than 10 birds or so just so that the ones that aren't at the top of the pecking order have a chance to eat and drink without getting hassled. That's what I would do anyway.
That is part of the reason I originally said (way back a bunch of posts ago )I wouldn't need windows up there because that whole eave line, along with the lower edges along the walls of the henhouse will have continuous vented soffit panels which I am positive will allow more than enough ventilation....the idea of adding the windows just makes it more appealing but I don't believe they are needed for ventilation purposes.
Buster, I have no idea how many hens .....I'd say around 20-25.