tab-width

tab-width is a variable defined in `C source code'.
Its value is

8

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

Documentation:
Distance between tab stops (for display of tab characters), in columns.
NOTE: This controls the display width of a TAB character, and not
the size of an indentation step.
This should be an integer greater than zero.

You can customize this variable.