It is entirely up to you. They are the ones who crossed a social boundary by raising a topic–your personal family planning decisions–that is none of their business. If their nosiness gets them more than they bargained for, that is their problem.
You can go on the counter-offensive: “Again? What is* that *supposed to mean? I suppose you think you get to have an opinion about how many children we have?” (It will not scar your nosy in-laws to hear this, trust me.)
You can go Miss Manners: “I *beg *your pardon?” (You repeat this and eventually change the subject without giving them the dignity of a reply, signalling that they have asked a socially-unacceptable question.)
You can be flippant: “You can’t have a basketball team if you space them too much. The young ones will be too short to compete, if you wait too long between kids…”
You can be an advocate for large families: “Yes, and we’re very excited. We want a large family, but not everyone who is open to having lots of children is blessed with lots of children, you know. Some people have really bought into the contraceptive mentality, too, but that is a whole other topic! Don’t even get me started!!”
You can be outrageous: “Well, yes, when both the husband and the wife have tremendous sex drives, the result can be a lot of pregnancies.”
You can take their nosiness as an invitation to ignore all social boundaries and put them on the spot with some probing questions of your own, instead: “Why do you ask? Would that make you feel uncomfortable? I’m always interested in what prompts some people to feel anxiety about other people’s decision-making. It is so fascinating to me…”