After scouring different resources, we found that experts have opposing opinions on this subject, with some saying that on average one nesting box is enough for five birds to use, while others saying that you should have one box for no more than 3-4 chickens.
Generally speaking, if you can afford to get more units, you should definitely follow the 3-4 birds guideline since it will help promote proper animal welfare. All in all, you are free to experiment a bit if you have the time to see which number works for you without overburdening your chicken nesting boxes.