It's usually recommended to provide one nest for every 4 hens, although they will likely only use one or two of them. A nest can be any of many containers, open in the front or on top. I have one that is an old plastic "bin" which is open on top and has a cutout in the front that makes it easy for them to step in; it is great for a broody and chicks. I have used kitty litter boxes, old wooden storage containers, and even cardboard boxes. People make them out of 5 gallon buckets; lay it on its side and fasten a lip across the bottom to help hold the hay in. I've never paid money for a nest box yet; there is always something lying around that I can use or modify slightly to use.