I think it is something you have to decide for yourself.
We only want one child for many reasons but none of them are financial. We are complete as a family now.
Having an only child is fine. But you have to provide for them the social stimulation they need both with parents and kids. If DS is awake then we are together (either me or DH or both of us). He only plays alone when he takes himself off to play by himself. We also go to 2-3 playgroups a week and he does daycare one day a week. Unfortunately we are temporarily away from family but when we were near them he saw his cousins once a week and grandparents at least twice a week. You definitely have to make a big effort. But I think you also do if you have more than one because they need to be able to socialise with people from different backgrounds and viewpoints.
I think someone else said siblings are a blessing and a curse. And I agree. While siblings can be best friends, or good friends, a lot of bullying can occur within families as well. It is definitely not all happy families just because there is more than one child.