Or the ever so lovely "are all those yours!?!?!?" But, my favorite is "don't you know how that happens?"
My wife was at the store with 4 or 5 of our 9 children and the clerk asked if they were all her children. She answered "no, the rest are at home."
My favorite is "you don't have a TV do you?"
When I was growing up, there was a tall blond woman in our neighborhood who drove a van (think Midge from That 70s show, and same timeframe). When they pulled up into a grocery store, it was like a Shriner's parade - child after child exiting the van. It was a hoot! I had large family envy when I was a kid.