I have two, a boy and a girl, and they've grown to be very close, even though they live so far apart, they are always there for each other. I never only wanted one child, and would've welcomed a third had it happened.
I'm an only child, and I thoroughly enjoyed my childhood. I had lots of love from my parents, and I know I was rather spoilt. Occasionally, growing up, I did wish that I had a brother or sister to play with, but I had lots of friends so that soon passed. By the time I was a teenager, I was actually quite glad that it was just me. However, I must confess, that the older I became, perhaps from my 30s onwards, I so wished that my parents had had more children. I'm in my late 50s now, and the past few years have been filled with some very difficult times regarding my parent's health, and they were actually very young parents, so it's not like they were really elderly, when you would expect problems. As an only child, you have everything to deal with yourself, with no brothers or sisters to support you, or share the burden of difficult times. Even though I have a close family myself, it's still very difficult and lonely not to have a sibling to share these times with. Having spoken to friends who are also an only child, pretty much all of them have said the same.
If I were asked, I would never advise my own son or daughter to have only one child, but I appreciate that every family is different, and only you can decide what's right for your family. But, I would say, think long and hard, not only about how you feel right now, but also about how your only child will feel during adulthood.