1 cup of Kosher or sea salt (no iodine) per gallon
I do an apple cider brine using cheap juice or concentrate, 1 pint molasses, 1 cup brown sugar, whole bay leaves, whole blk pepper, cinnamon sticks, nutmeg, and whole cloves. Now you can heat this all up, it will help bring all the flavors out if you do, or just mix it up and go. I do both depending on my schedule.
Some ask why the molasses, well it gives a nice flavor, plus it will help brown the skin some. It doesn't darken the meat just the skin.
I will normally do 6 or more at a time since I have a big smoker. I dump it all in a 5 gallon bucket 2-3 gallons of brine and fill it with chickens using a weight to keep them under. Let this go for 12-24 hours. I will normally go 24 for better flavor but really only 6-8 for the brine to take hold. I will remove them, tuck the wings, truss the legs through the fat, allow them to drain 30 minutes or so and put them on the smoker.
Normally when I smoked like this it was in the winter/fall when temps were pretty cool. I partially thawed my chicken too. I opened the cavity and removed the gizzards bag/neck and while they were still stiff I put them down into the brine. I just covered the bucket and left it in the garage. It would still be cold the next day as the salt help keep the temps low.