I have for a long time believed that kids with many siblings are happier and better-adjusted personally and socially, and that these outcomes increase with the number of siblings. Are there indeed any studies or other evidence indicating either correlation or causation between these factors?
My perception is that a child's social adjustment is more complex than just how many siblings they have. For example: there are articles discussing how Only Children are More Successful while you can find evidence to support the contrary. I believe most of these types of articles are anecdotal generalizations in both directions, though.
Children are complex and have their own personalities - no two are alike and therefore family structure, parenting styles and the child's own choices will all vary from family to family.
In the same light as your related link accepted answer mentions: What is more important for you is understanding what you want for a family. Study how you want to parent regardless of child count and become comfortable with the idea that parenthood is a wild adventure regardless of how many children you have.