I'm an only, but I've got older step-siblings, so it's kind of the best of both worlds. I had Mum's undivided attention when I was with her and Dad's when I was with him.
Mum was a single parent from when I was 4 until I was 12, so I'm pretty independent and self-sufficient because there was stuff she had to get done, so I played by myself when there were no convenient friends about. I wanted a sibling, but accepted that it wasn't going to happen.
I've never been exactly close to my step-siblings, not in the same way they are to each other, and in a way I wish I'd had that. We're all good friends, but there's an indefinable closeness between them that I'm not part of, and I feel a bit sad about it.
When my Dad was ill, it was down to me, and that was hard because I was 100 miles away and self-employed, but I did as much as I could (nearly wearing out my car in the process!) and just got on with it. There was no room for any arguing about what to do for the best as there was no-one to argue with. Equally, there was no-one to argue with about who was or wasn't pulling their weight. The one thing I might have liked in a sibling is someone to share memories of him with.
DP's got a brother, who he's close to. He wouldn't want it any other way, although they fought like cats in a sack as teenagers. Our "plan" is to have two, subject to what the fates decide. As yet we don't have any, so it's not a done deal.
I fear I've gone on a bit, but the point is that I'm happy with my lot, with a touch of grass-is-greener wistfulness rather than any real dissatisfaction with it.