I have 16 plus a roo in that exact same size. We do free range every day but their doing fine no fighting or feather picking.If your going to keep them locked up I'd probably 3 or 4 just to make sure I had enough room.
Since yours will be confined to the coop/run all the time, it's good that you have plenty of space. The smaller the number of birds you have based on your available space - the better! Happier chickens for not being cooped up and packed in. And easier to keep clean
When you cut your grass, you can rake and toss that in there too!