people recommend keeping them lower to avoid injury.
I can see two different factors here. If you have large breeds and feed them in a way to make them really big for their breed, they may have problems with leg injuries if they hop down. This is a real issue for many people that breed for show, they get rewarded for meeting certain weights with their birds, typically weights heavier than hatchery birds get on "normal" feed. Some people like to feed their birds a rich diet so they can get big. I don't feed mine that way and don't have any issues with my full-sized fowl birds flying up or down from my 5' high roosts.
The other potential issue is that the higher the roost the more clear ground they need to land. In your coop that shouldn't be a big problem but watch where you put any waterers, feeders, nests, brooders, and such.
How many nest boxes?
For 24 hens I'd go with 6 nests. They probably won't use all of them, especially all the time, but you need a number and that will be enough.
I plan to have a nursery in there as well
I assume this means brooder. I'll show what I did. The brooder fits under my roosts with the top acting as a droppings board.
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Then you have my juvenile roost. It's a horrible design but that's what I get fro it being an afterthought.. I integrate a lot of young chickens and they don't sleep on the main roosts with my adults until they mature, so they were sleeping in my nests. I put this roost over my nests to give then a place to go that was higher than the nests but safe from the adults. The top of my nests are droppings boards but are a pain to scrape. If I had it to do again I'd put a flat droppings boar all the way across.
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