I think there are two questions (not that you asked, exactly):
Adoption or not?
How many?
The benefits of adoption are many. There must be thread here about adoption, mustn't there? We spent a month in Vietnam to pick up our daughter, and it was the among the best months of my life. Would be thrilled to answer PMs about adoption, as I don't have much time to post on that now.
As far as how many, I would vote one. I'm shocked there aren't more people that say that. With one, you still have some semblance of your own life. With two children, that is gone. This is validated by observing our many friends with one child, and the friends with more than one.
As far as the siblings to play with issue, it's far over rated. Many siblings DON'T play with each other. And many single children (like ours) have many many friends. There isn't a day that goes by that there isn't contact with other kids.
The best of both worlds!