I am probably one and done. One thing I frequently see posted on MN, is people saying that they had a second child so the first child would "have other family" once they and their partner are gone.
I think this is nice if it happens, but it's an overly optimistic and romanticised view of sibling relationships as adults.
I have a brother, and he adds very little to my life, not in a nasty way, but in a practical sense. As an adult, I may as well be an only child.
Our parents have always taken great pains to treat us equally, so there's no golden child scenario to create a sibling rift, nor has there ever been a dramatic falling out, my brother just doesn't really give a toss about staying in touch with me.
I am a couple of years older, and we got on as kids until he hit his early teens, when he decided to reject all things academic. His choice, and it's fair enough, but it has meant that he and I have had very different lives and interests since then.
As adults, he has never contacted me to keep in touch, so texts to maintain any semblance of a sibling relationship always come from me, and he just generally can't be arsed.
He's now moved with his partner to a country in Southeast Asia, and doesn't have any interest in asking about his nephew (my toddler DS), or in keeping in touch with me unless I message him.
This context makes me scoff a bit when people say I should have a second DC to guarantee a lifelong family member and companion for my DS.