We got over 20k at home for FC Twente and PSG so I'd say 25 to 30k for Copenhagen
City tend to fill the lower tier first, but I think there were some away fans in the Upper Tier in previous Cup games. I can't quite remember
I remember Omonoia's Green Army jumping up and down all game (and before), but that was in the Lower Tier. They made a lot of noise which puts to death the idea that you can't get an atmosphere at COMS. In fact I think it's the other way around. The new stadium is more atmospheric than Maine Rd. If you were at the back of the North Stand at maine Rd, it sounded deafening if you were there singing, because the roof was low. But if you were in another stand you could barely hear it. At Coms, if you're singing, it doesn't sound that loud, but the noise carries better around the ground, probably because of the roof all the way around the ground