I was perfectly happy having just one. My older daughter is, and has always been, mature, motivated, sweet. She's basically the perfect child. No discipline issues, has always done well in school, has tons of friends. Well, my husband was insistent that we have a second child. On and on he went about how selfish I was being to our child in not giving her a sibling. I began doubting myself and in the end, agreed to have a second child.
My second child is a happy, beautiful girl, and she's extremely bright (genius-level IQ). However. She is a handful. She has ADHD-Combined, and because she basically can't function in a normal public school setting, I now homeschool her. Let me tell you, if someone had told me what my life would be like today, before I got pregnant, I'd have stopped with my first child. Don't get me wrong, my younger one has such a hilarious personality, and I love her just as much as my older daughter. But having a special-needs child is a huge drain on your life in almost every area you can think of. It is *exhausting* dealing with her day in and day out.
I only tell you this because I think you might feel like I did - that you are happy with one child and don't see any great need to have another just because people expect you to, or want you to, or whatever. All of those people who want you to have another will be NOWHERE when you need a break from your special-needs child. Even your husband! And all of the people who said, "Oh, but your kids will grow up to be best friends! They'll have each other when you're gone!" Umm, well, I guess things might (drastically) change by then, but pretty much my older child has never loved her sister. We are now at the point (they're 12 and 9) where the older one is occasionally civil to her sister, but that's about the best that it ever gets. Most of the time, they scream at each other, yell things like, "I hate you!" which gets all of their privileges taken away, not that it stops them the next time they're fighting... good times. Yeah. Don't have a second because then your first will have a friend. Siblings are just like anyone else on the planet - they have their own personalities, and if your kids' personalities don't mesh, it's not fun.
Sorry, I wish I had something more cheerful to say about my two, but that's what I've got. Individually they are great kids in their own ways, but together, I realize I should have stopped with one. Not that I'd trade in my younger daughter for anything, but... yeah. That's my honest opinion.