I haven't had time to read all the replies, but IMO, you can't plan your family structure around how you think/predict/worry/hope your children will feel about it. Because you just can't know. You can see it on this thread -- some people liked it, some people hated it. And if you polled MDC for "Adults who grew up with one sibling," or "Adults who grew up with three siblings," etc. you would likely find the same thing. You just don't know what your child's or future children's temperaments will turn out and what will happen between them, or between them and you, to create the attitudes toward your family that they'll carry into adulthood. And no matter how a person is raised, that person will always wish something had been different. So I would make this decision based on whether you
want to have multiple children rather than on how you think your existing child will be affected.
That said, I was raised as the only child of a single parent and it was fine. There were times where I wished I had a brother or sister, but I'm pretty sure if I had one, there would have been times I wished I didn't.
I think kids will always feel a little bit of the "grass is greener" no matter what the situation is. Bottom line, it was what I knew, so it felt familiar and right most of the time. As an adult, I can see intellectually how it might be nice to have more people in my family for support, understanding, and fun, but it's not a desire I really feel
very deeply. As I said above, I think everyone probably has some regrets about how they were raised, or issues that stem from how they were raised. People have probably already listed some of those issues for onlies. But I honestly don't think they are any worse (or better) than the issues that come up for people raised with siblings.
So again, my bottom line is, do what you feel is right for your parenting capacity, not what you feel you should do for your existing child.