An indentation is to be one-half inch, which equates to approximately five spaces. However, technically speaking, an indentation is governed by its measurement and not by a space count.
The primary reasons spaces technically are not the primary measurement of an indentation rests on the fact that spaces slightly vary in size depending upon the font setting at a particular time. Spaces adjust to some degree to accommodate font size. With a smaller font size, the number of spaces, and fractions of spaces, needed to reach the one-half inch mark increases. Similarly, the space ratio declines when a font size increases.