I am one of 3 and have always wanted to have lots of children. DH was an only child but grew up with his 2 cousins who are all close in age, so that was almost like having 2 brothers for him.
SS is an only child and now he is 13 you can tell that he has had quite a lonely childhood. He has always had kids around him in school and after school club, but sadly playdates during holidays etc were never encouraged when he was younger. I think he has had some issues making friends at school over the years too. Because he is an only child (and until recently the only child in the family) he got all the attention from the adults in the family, and that has almost made him a bit cocky. When he meets new kids I think they find him a bit too much at times and so he has trouble with this. DH has spoken to him about not being so overpowering with people, but it's just the way he has been allowed to grow up really.
If we go on holiday he always wants to find someone young to play with in the pool etc, and most of the other kids are all playing happily with their brothers and sisters. I think he has found it harder as he has gotten older. He knows that we are wanting to have children and i do worry that when he sees his half siblings grow up together that he won't feel a part of that bond due to the age gap and the fact that they will be growing up together, whilst he will just see them once a fortnight. And as I mentioned he has always had attention from all the adults in the family (but now another grandchild has been born), but when we have spoken about having children he says that he won't get as much attention from people / not as many presents etc etc and that he won't like that! I understand that it is natural for kids to feel like this, but I would expect that worry to come from a younger child as opposed to a 13 year old!
If I have a choice in the matter I will definately want more than one child. Personally about 3 or 4. Everyone is different though so it's all about what fits in with you and your life.
xx