I am not a parent but I am the oldest of nine and I can share some witty comebacks my parents have. Also, I can share some of my own (I am confronted quite a bit when people think the youngest 3 are my kids).
When someone asks my dad, "Are ALL those yours?" He always says, "If you think this is too many, you should see the ones I left at home!" My mom likes to say,"No, I picked up a couple extra in the produce aisle/food court/etc..."
When someone says, "You sure have your hands full!" my mom always says,"Yes, and my heart is too."
My mom gets told often, "I can't believe how you do it! I can't even handle my two!" She says, "Oh, I just do it. Wake up in the morning, go, go, go, and go to bed when its done." However, I know she is thinking, "Yes, you don't strike me as the kind that could handle more than two anyway. And if I had kids like yours I couldn't handle more either!"
When someone asks me, "How many more do you think you'll have?" I turn it back around on them and say, "How many more do you think?" This tends to make them go away. When my mom gets asked this she says, "Well, we still have room in the van for 1 more!"
Of course you'll deal with the question, "Haven't you heard of birth control?" We all say, "Oh yes, we know what birth control is. Its for people who don't want children."
One more: "Do you get any time for yourselves?" My dad says, "Obviously we get some time, or else we wouldn't have all these kids!"
Hope this helps! :)