Having "She's thirteen cats" as the default "correct" answer sounds archaic. At least in the US, "she's" is hardly ever used to denote possession and sounds unnatural.
I don't know that I've ever seen it alone denote possession (vs attribute). She's got, she has, that's hers. So that looks like, she is 13 cats. She's a ginger. She's tall. She's purple. She's the mother of twelve children... She's a polyglot.
(They've changed it anyway).