c-basic-offset is a variable defined in `cc-vars.el'.
Its value is

  • This variable is safe as a file local variable if its value
    satisfies the predicate `integerp'.

*Amount of basic offset used by + and - symbols in `c-offsets-alist'.
Also used as the indentation step when `c-syntactic-indentation' is

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.