I've seen this question regularly on MamaSource so there are plenty of people wondering the same thing--to have more or not.
Our son is 13 and recently said, "I love our little family!" He used to lament that he didn't have a sibling. However, as he's been around older or younger siblings of friends through the years, he's noticed there are some nice things about being the only as well. In his words, siblings can be annoying at times. Sometimes they're great, but sometimes it is special to have more time to craft as you like--whether alone or with friends at our house.
I had 5 siblings, my husband has 3. I was part of a larger family, but I also had an only child experience in some ways. There were 5 kids in 6 years, and then I came 5 years later. I felt like I had the best of both worlds! Sometimes I felt lonely, but often I enjoyed the quiet. Sometimes as adults, I love it when we get together, and sometimes I can't believe the noise! Is that because I had my parents to myself for six years or not (my next sister was a year ahead in school)? Who knows?
When our son was little, I always helped coordinate play dates or was part of play groups. I'm a big believer in surrounding yourself with people you enjoy or that help you grow. For some people that is their family, for others it is their friends, for many it is a mixture of both.
Because we were not able to have another child, the decision was taken out of our hands. We were almost 37 and 42 at the time of our son's birth and it never dawned on us that, if we had one child, we might not be able to have another. We now know secondary infertility is common. We tried fertility steps for a bit and then decided it wasn't worth it for us. One was a delight and we decided we were comfortable with stopping there.
There is a part of us that will always be a little sad about not being able to have another child, but it has been a delight to have one, and only one, as well. I can take him somewhere and have some wonderful one-on-one time with him. Or my husband can do the same. We are able to do more things with him because it costs less for a family of three than for more. He has learned to deal well with adults as well as kids because he is around both large amounts of time. He has lots of opportunity for sedate times and rambunctious times. (Sometimes we're tired and sometimes we're silly.)
But cost or quiet would never be a reason for us to determine the number of kids. Whatever the number, most people can make it work. Having more kids has lots of advantages, too. The older kids can help the younger ones. They learn to deal with daily struggles more directly with siblings. There are more kids to help when the parents get old.
But y'know, it is such a personal decision for each family to decide. Our neighbors across the street are the same age as us, but they have five kids. Our son was just like a younger brother to them. He has friends who are onlies as well, and other friends who have 3, 4, or even 6 kids in their families.
When we decided to stay with just one child, we asked several people who were onlies how they liked it. Many liked it; a few were lonely and hated it. But in the end, it was our decision. I wish you the best as you decide and that you are at peace with the decision either way!