4 if you can get them to go into them. mine like 55 gallon drums or any spot that is covered and hid from sight. They never nest within 4 or 5ft of each other.
In case you were hoping for ducklings..... The chances of them all going broody at exactly the same time are slim..but hey...stranger things have happened. One for each is what I would give them just to make sure though. They may favor some over the others- or share- but at least with one each you can move them if you need to. Make the boxes far enough apart that you can section off each mum for a while when hatching to protect the babies. If you are just wanting eggs... 2-3 might be enough- They may choose their own place to lay anyway.
