It can never be 8 countries. It was 6 countries. Serbia, Montenegro, Bosnia and Herzegovina, North Macedonia, Slovenia, Croatia and Jetpunk recognizes partly recognised Kosovo.
As for the former republics of Yugoslavia, you are right, it is 6 countries. Regarding Kosovo, I have never taken a quiz that doesn’t recognize it, though I am active here since 2020. Lastly, it can be 1 more, because the instructions say that it has to be current countries that possesed a territory which belonged to Yugoslavia across History: it is not 6 countries, but 8… I understand that normally it is 7, but this quiz is a bit different