Is it about what I want?
I mean I want many things, such as, for Law & Order to stop doing episodes with character drama and start solving cases again; getting a sitter and seeing Prince Caspian in the theatre. "Want" like air, fills a vacuum. If I think about my family years down the line, I imagine more kids than I currently have and I like the vision that I see. So in a sense I could answer the question (um...one kid every two years or so...until my wife is about so old...so my answer is...).
But honestly, what bearing does that have on anything? More than wanting children, I love children. I love the children that I have and I will love each and every child that God gifts me and my wife with. What else matters.
(Incidentally, I'm with him on the Law & Order comment, too.)