Is "nohou" in intrumental here? Why? I thought that "kolik" needed genitive O.o.
It is genitive (dual) https://en.wiktionary.org/wiki/noha#Declension .
"How many feet" is not accepted?
I am not sure flies have any feet, but one usually speaks about legs in insects... And
But even if they do I fear they are not called nohy in Czech, they would be chodidla or something like that.