n

    The number that the next .CHAPTER will have. Entering the .NUMBER
    CHAPTER command without an argument gives Chapter 1.

 +n

    Adds n to the number of the most recently issued .CHAPTER. The
    result is the number that the next .CHAPTER will have.

 -n

    Subtracts n from the number of the most recently entered
    .CHAPTER. The result is the number that the next .CHAPTER will
    have.