fill-column

fill-column is a variable defined in `C source code'.
Its value is

1000

Original value was

• Automatically becomes buffer-local when set.
• This variable is safe as a file local variable if its value
  satisfies the predicate `integerp'.

Documentation:
Column beyond which automatic line-wrapping should happen.
Interactively, you can set the buffer local value using C-x f.

You can customize this variable.