“Oh my, are ALL those kids yours?”
"OH MY GOODNESS! Where did the other seven go?"
Either that or,
"HOLY TOLEDO! That dream wasn't a dream after all!"
She might try saying something along the lines of "we really didn't plan on it, it's all the tossing and turning my husband does in his sleep"
I'm #9 out of 13..... my Mom used to say, "dingdangity stork just kept bringing them." and shrug. But then, Mom only had 3. 4 are steps, 4 are fosters, and 2 are my Father's kids. But we all grew up as siblings, and we were ALL my Mom's kids.