I'm not exactly sure what you mean by roosting area. They do not roost in an area, they roost on roosts which are linear. I don't believe in hard and fast magic numbers for chickens, there are way too many variables. One of those variables is personality of the individual chickens. Most get along fairly well but some are just brutes and bullies to any chicken around them. They can change the dynamics of your whole flock. What works well for me might not work at all for you. I don't give guarantees on the behaviors of living animals, you never know what one will do.
I'm guessing your roost area is 8' long. If your 20 chickens are all the same age and are fully integrated, the minimum I'd go with is two roosts 8' long each. I'd set the first one at least 12" off of the wall, a little more is better but 12" will do. The next one should be at least 12" from that one. In most cases that will handle 20 chickens but if you wind up with a brute or two, you could have issues.
But there are other considerations. Chickens don't necessarily take up a lot of room when they are on the roosts, but they need to get on the roosts. That means clear area to land when they fly up. The roosts also need to be higher than the nests or anywhere else you don't want them sleeping. The higher the roosts the more room they need to spread their wings and fly to get up there and, equally important, the more clear room they need to fly down in the morning. If they bang into walls, nests, feeders, waterers, or other things on the way down they can injure themselves. If it is that crowded they may not even attempt to fly up.
We use all kinds of different configurations in our coops. Some are so crowded and jumbled the chickens cannot fly so they use ramps or intermediate steps so the chicken scan walk or hop up and down. Some people use ladder roosts instead of roosts the same elevation. The roosts are only one component of the coop. What the rest of the coop looks like will have an effect on what you can do for roosts.