Wow, what an interesting thread! I'm pregnant with my first (and I have only ever wanted 1!) but am already sick of people commenting about the next "few" ?!?!
I am an only child, and cannot believe I would have wanted siblings. I learned to share, learned to use my imagination, grew up to be confident and secure. My Mum died when I was 12 after 5 years of suffering with cancer, so I did not get to spend as much time with other people my age but still learned a lot about life.
Incidentally, my Mum had 2 sisters who lived very close by and who could have made a difference about the 'family' life as some people have mentioned but they did not; instead they chose to ostracise me and make my life very uncomfortable, until I took the power away from them.
My Dad has been both parents to me and I dread the day he dies, but I know I wouldn't feel any better if I had a number of siblings to share it with. May get slated by some, but perhaps I might not enjoy sharing the grief with people!!
My DH wants more than one; he is Irish and has 3 siblings, which IMHO have caused nothing but strife within the family - he constantly gets berated for not being as good/clever etc etc as his siblings even though he is the only one to strike out and make himself a life away from home (perhaps why he gets punished!).
I really think you have to go with what you want; here and now (obviously with some planning for the future) and what your instincts tell you is right for you