My flock runs in the mid to high teens and I have an acre of mostly grass in west southern Oregon, so I'm guessing same type climate. My birds are out all the time, I have an old greenhouse for them to sleep in and they like to lay in the hay barn. I was blessed to have a dog who hated intruders in his territory (coons, skunks, etc) and couldn't care less about the chickens. Sadly, no more, so I'm more alert to predators now. Anyway.........I do supplement feed them and see my production fall if I don't supplement them. I'm in the chickens mostly for the eggs, so I'm willing to feed them.
If you're wanting to give it a try, I'd start with ten or twelve. A good roo to look over them and make sure they have cover and a good place to sleep. Another thought is, are you gonna want to egg hunt over two acres? Hens lay where they want, and will frequently move their preferred spot.