To what camp were the prisoners marched in Night?This answer should be somewhere in pages 21-43.

In the section of the book to which you are refering, the prisoners are marched into the camp at Auschwitz.

Eleazer and his family are transported from Sighet by freight train to Birkenau, which is the reception center for Auschwitz.  As soon as the prisoners are taken off the train, they are separated into two lines.  The men are instructed to go to the left, the women to the right.  Eleazer watches as his mother and little sister Tzipora head off to the right as instructed.  It is the last time he will see either of them.

Eleazer walks with his father in the line to the left.  The prisoners are ordered to form groups of fives.  One of the veteran prisoners advises Eleazer to tell the SS officers that he is eighteen when he is asked his age.  He also tells Eleazer's father to say he is forty rather than fifty.  It is of utmost importance that they convince the soldiers that they are fit enough to work, otherwise they will be "turned into ashes" in the crematory. 

When it comes time for Eleazer and his father to face the SS officers, they lie about their ages as instructed, and tell the soldier that they are farmers so that they will appear to have useful skills.  Their ruse works, and they are marched away from the ovens towards the barracks at the infamous work camp at Auschwitz (Chapter 3).