The nominative plural is выходы; why does this phrase use the genitive singular?
Because два, три, and четыре "take" genitive singular; шесть-девять "take" genitive plural. Incidentally, this applies whether these are stand-alone numbers like here or at the end of a longer number, like 1,000,002.
Thank you. I do recall reading that earlier in the course but I was overwhelmed with all the rules and that one didn't stick.