The number of boxes doesn't equate number of chickens, necessarily. It depends on the amount of space you have in the coop, a good rule of thumb I've heard is between 3-5 square feet per hen, depending on their size (3 for bantys, 5 for heavy standard breeds). However, it's a good idea to have 1 box per 2-3 birds, in my opinion. About the risk of free ranging, there is always going to be a chance of predators no matter the circumstances. Some breeds are particularly good for evading becoming dinner, but I can't remember which ones. Do a little research and you should find more info.