Well, there's a couple ways to answer your questions. First a fox is the likely culprit. They are very good at digging and they take a lot of birds and only usually leave feathers. But without more information, that's what I would guess.
As for your first question, there's a couple ways to answer it. Chickens need space, that's really important. A general rule of thumb is 10sq feet per bird in the run and I think it's 5sq feet per bird in the the coop. So if you have 10 birds you would need a 100sq run and a 50sq foot coop. But the sq foot per bird for coops I'm not positive on, that's just what I go by. So you need to make sure you have enough room for any birds you plan on adding. Another thing you need to consider is size and pecking order. It's a bad idea to add chickens that are smaller to the current members of the flock. Mainly because the bigger chickens have the advantage of size and they're going to try and let the newcomers know that they're the boss. Get like sizes and numbers. Don't get one chicken at a time and merge it into the flock. Make sure the newcomer comes into the flock with the friend so it's not getting picked on by every other chicken.
