Single child here, I didn't feel as demanding; however I certainly had a friendship-style relationship with my parents that all my other friends (none of them single children) didn't have with their parents.
In my experience, usually one child of a set seems to always be a lot more demanding than others.
The single children where very demanding on their parents. The only source of attention and entertainment is the parents, so this makes sense.
Two children entertain themselves a bit more, but like in duopolic situations you have the most extreme competition. This leads to a lot of fights over shared resources.
With 3+ children fights over resources between 2 children don't lead to zero sum gains, so that the competition actually becomes less. Cooperation becomes more important. And the kids almost always entertain themselves.
I'd definitely say the 3+ children parents were usually the least stressed, but maybe that was just my impression. The older siblings at some point functioned as quasi parents to the younger siblings.