I have 28 hens I get 8 eggs so far and most are all in one box. They pick their faves and leave the rest alone. 6 is plenty and if you need more space you might be able to reduce with only 8 hens.
I got them at Lowes via @brounii who had them first. I love that they self stack.Almost 16 x 12 x 19 plenty big enough for more than one at a time. Less than $10 each and delivered to the house too...