I've found that of en route sectors in the US, the ultra high sectors (FL350 and above) you can pack many aircraft into them since not much is said except hello and goodbye. But our one departure sector where I currently work has 5 SIDs and numerous satellite airports an is mostly like a Chinese fire drill trying to manage the frequencies during pushes. And as others have commented, mix in some weather,bad rides, military activity etc, you can go down the tubes with a small amount of aircraft.