I'd give them a try before clipping their wings. They are fairly hefty birds and may not fly out. Clipped wings reduce a bird's mobility somewhat and along with that their predator evasiveness. If the pen is small enough, a top would be sensible and aid in protection from predators.
There is a big difference in what they can do and what they will do. I've had chickens fly out of a run with 5' high fencing. I've contained chickens in 4' high electric netting and never had one fly out. Once a hen got out through the coop door. While I was trying to herd her back toward the coop door, she flew over the 4' high electric netting to rejoin her buddies with at least a foot to spare. Mine have absolutely no problems jumping/flapping to a 5' high roost. I have no doubt they could go a lot higher of they wanted to.
Age, size, breed, all factor into what they can do. But what they will do depends on their motivation. Some chickens like to roam and explore while some are stay-at-homes. Some will get over the fence to get away from an amorous rooster. The bigger the run, the less likely they will want to get out. You never know what they will do. There is no magic number that will definitely contain them, but the higher you go, the better your chances.
One thing that can happen. Chickens like to perch. If you build a fence with a top rail, they could very easily fly up there to perch and who knows which side of the fence they will jump down on. Once they are out, they forget how they got there. They don't go back in the same way. I suggest you make the top of your fence wire, not something solid they can see as a place to perch on.