Long long ago, Andy taught us not to ask personal questions, like "When are you having another child?"
Today a friend of mine lets me know asking questions about somebody else's adoption is personal, too.
You know, just in general, if you have to ask, you probably don't know the person well enough to be asking. When you find out something about somebody ("Oh, you adopted one of your children?") it may seem polite to make conversation about it, but remember that it is not a novelty to the person you're talking to, they've heard everything you can think of to say before, and they may not want you prying into details. Especially if they actually say, "I don't want you prying into the details."