What size nesting boxes are you building? A normal recommendation on this forum is a minimum of 12” x 12”. That normally comes with the recommendation that Judy mentioned, 4 hens per nest. Even with the 12”x12” boxes, since you have more than the average sized flock on here and they tend to lay in just a few anyway, I’ll agree with Judy. Six should be enough for you.
I personally prefer larger nests though. I made mine 16” x 16”, mainly because that hit my stud framing really well which made them easier to build. I’m glad I did. It’s not that unusual for me to find three hens crowded into one nest even though there are other nests empty. That’s with just 7 or 8 hens and three nests total. A larger nest allows them to share a nest without one hen literally sitting on top of another. There are photos of that on this forum since it is so cute but my preference is that they can all sit in the bottom of the nest, especially in summer when it can be really hot.
There is another reason I like the larger nests. I once had a broody hen hatch chicks in a cat litter bucket. Not a cat litter pan but the bucket the litter comes in. That’s fairly small, somewhere around 8” x 11”. They will lay in something that small. But the first chicks that hatch like to climb on top of the hen and sit on her back while waiting on the later ones to hatch. In that small nest the broody was so close to the edge that the chicks sometimes missed the nest and fell to the floor. I put a chick back in with Mama three times that hatch and then retired that nest. Most 12” x 12” nests will be big enough for a broody to hatch but that depends a bit on how you build them. I just prefer they have plenty of room.
You could build what is called a community nest box. You can do a search on those if you wish. They can be different sizes but the idea is to build a large dark box, maybe 24” x 48”, with openings on the ends. The top is hinged so you retrieve the eggs by lifting the top. It’s not that good for a broody hen to hatch eggs in but a community nest like that can supposedly handle more than 20 hens by itself. There are all kinds of ways to build nests. The larger they are the more hens they can handle.
This depends on how you build them, open on top or closed on top, but consider the opening for them to get in before you start cutting wood. I like a lip around the bottom of a minimum 4” to help hold the bedding and eggs in. Many hens like to scratch around before they lay, getting the bedding just right. If the lip is too low they can scratch out bedding and eggs. I normally make my openings about 8” wide and 8” high when I close in the front. That means the nest needs to be 12” high. But consider how you frame it. If you have a framing member going across the front, especially up top, you may need to go even taller to keep the lip and opening. My nests are typically 16” high to account for all this. I find it really beneficial to draw all this out, with dimensions, before I start cutting wood.
One last thing. People use about anything to make nests. They don’t have to be made out of wood. Some popular items are large buckets you can probably get for free at a local deli or bakery and cat litter pans. Some people just set a cardboard box on the floor filled with bedding. Some people make them out of old furniture like a bookcase or chest of drawers. I even saw a photo on here where someone used an old kitchen sink. You are only limited by your imagination as to how you make the nests or even what you use. These threads are fairly old and are missing some photos but you might scroll through these to see some things that people have used to give you ideas.
https://www.backyardchickens.com/t/41108/show-us-your-nest-boxes-ingenous-design-post-it-here/220
https://www.backyardchickens.com/t/...-your-creative-nesting-boxes/80#post_12395882
Opa’s Rollaway Nest Box
https://www.backyardchickens.com/forum/viewtopic.php?id=287684
The big problem with nests is not that they have to be built a specific way. The problem is that you have so many options on how you can do it that it is sometimes hard to make up your mind how to do it. I probably haven’t helped you much in that aspect. Good luck!