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Hope you enjoy your stay with us!!
You've already gotten some good advice space-wise. If you plan on extra room, (the 4 sq ft/bird or a little extra) you can add in a few extra birds if all things go well and not stress either the birds or yourself. I see you are in a relatively warm area, so your birds won't be locked indoors for days on end due to blizards and such, most chickens will go outside in rainy weather, and don't mind cold of the sort you'll be getting, so you should be pretty ok. so, if you want 20 birds, and you build a coop that's say, 8x10... that's the 4sq ft. of course, you'll need some room to store feed and supplies, so don't forget that... The amount of room or land you have will determine how large you want to go.
I'd bet that if you put in more than 4 nest boxes you'll have lots of wasted space. They'll probably all pick one or two boxes and wait in a line to use the same ones.
Ten hens to one rooster is generally considered a decent ratio, but that's not a Rule with a capital "R". It's sort of a guideline. Some flocks have much higher ratios and do fine others have lower. It's a decent rule of thumb to go by though.
Your breed choices determine some things about your set up. Standard or larger hens need lower roosts, banties can fly very high up, large birds can hop up to higher roosts, but can hurt themselves coming down if the roosts are too high. They're heavier and clumsier. The two types can be mixed together usually, but since they have different preferences, you just need to be aware.
If possible you may want some seperate area to put a broody hen, or an injured bird as well as some place to put young birds that aren't ready to mix with older chooks yet, but can be outside. If you are going to have a run, it's kinda nice (not necessary) to have two sections or two runs so you can seperate them at times. There's lots to think of, most aren't requirements, chickens generally do very well with the absolute bare necessities, food, water, dry shelter.
Here's a chart that has lots of info about breeds. Everyone will tell you what they like, but each situation is different.
http://www.ithaca.edu/staff/jhenderson/chooks/dual.html
//edit// I also have had great experiences with Meyers Hatchery in Ohio.
http://www.meyerhatchery.com/get_dept_1.htm as far as I know all the ones mentioned so far are reputable busineses and are good to deal with.