I am considering having only one. I am ttc as a single person, and finaces will be an issue of sorts. If I have a second child, I assume it will be at least 4 years after the first, maybe 5 years. I just can't plan that far ahead until after the first, you know?
I don't think an only would be lonely. I have very close friends who are planning on starting to ttc this spring/summer also, so hopefully our children will grow up refering to each other as "cousins", and we will see them on a daily basis.
I also will have other close friends moving down here who might become parents in a few years, too - I have a wonderful feeling that there will be a nice balance between alone time and time spent with other families with children that we will consider "chosen family".
I think it'll be nice to have one
Just me and him/her - a mother/child - teacher/student - mentor/mentee relationship is what I'm hoping for (but who will teach whom is the question!).
I also feel some guilt over contributing to the over population of our lovely planet, but not enough to stop me from having a second if I really want it a few years down the line. If I do have two, I want there to be 4-5 years between them so that each can have the opportunity to have one on one time with me as an infant. I'd also like to foster teens sometime down the line... there is such a need.
I do however, really enjoy having two sisters and can't imagine what it'd be like without them. When our parents die (hopefully many years from now) I think it would be a really scary feeling being alone (we are not particularly close to extended family).