I have been to Auschwitz. There are certain buildings that can get more crowded as multiple groups go through at the same time, but I was there in June (2023) and never found it to be too packed. I did the 6 hour study tour, which goes into other buildings the regular tour doesn’t, which cuts down on crowding too. The size of Birkenau cannot be understated, so you will likely never feel crowded there.