I have 4 DCs, but we are very close to a family who have an only DS.
My DS and their DS are very close, and when he's at our house it feels very much like he is part of the family.
He is much more confident than any of mine have ever been, and relates extremely well to people of all ages.
I know that when he comes to our house he loves the hustle and bustle and when my DS goes to his house they have the best time ever.
I wouldn't swap my situation for the world and neither would my friend.
My DC1 (DD17) also has a best friend who is an only child, and she is one of the loveliest girls I have ever met.
I know a few other onlies and they are a bit wimpish and wet tbh, you can spot the children who are spoilt and believe they are the centre of the entire universe a mile off.
This is more likely to happen if you're child is an only, ime and imho, but it is by no means guaranteed. As I've said, some of the most lovely children I know are only children.
Having what I thought would be an only child for many, many years (and feeling desperately sad, like there was something missing, tbh,) I have seen this from both sides.
Mine drive each other mad, and squabble a lot but they also have great fun and are very good friends.
I think all the concerns and sadness I had regarding my DC1 being an only child was mostly in my head, she wasn't aware of it and I don't think it affected her. The problem and sadness was mine for her and myself, but she did not feel that sense of something missing IYSWIM.
For those who have only children but not through choice, I would say that try to keep a lid on the sadness, I have friends who will never come to terms with the fact that can only ever have one, and this has affected their child big time IMHO, though I don't think they are aware of this at all.
I hope this doesn't sound patronising or arrogant, it's just that, having been in the situation of desperately wanting more, and thinking I would never have more, I can now see that only children are actually in a very fortunate position in many ways, and they are unlikely to feel that they are missing out unless they have parents who are perpetually in a state of sadness brought about because they can't have any more.