Our DD will more than likely be an only child unless a mircle occurs! I had a very hard end of my pregnancy and an awlful labour and delivery. The result is a damaged uterus that may not carry more children. Prior to all that though DH and I did want a "biggish" family. However we can see now that it isn't for us, Asides from the pregnancy issues, my bipolar didn't cope as well as I thought it would with pregnancy and postnatal so we are happy with the gorgeous little one we have!!
I don't feel she will miss out on anything. She has plenty of chances to socialise even now. Although I have a sibling I have never been close to him and even now as adults we are lucky to talk once a month if that.
Anyway my 2 cents. I see no problem in having one child if that is what is right for you and your family.