The numbers of people measured are important as we know that data obtained in measuring human dimensions is normally distributed (inverted u-shaped curve) over a given population.
The normal distribution allows a range to be used to design for as many individuals as possible.
As an example, the difference in height in the UK between the 5th percentile female (151.4cm) and the 95th percentile male (186.9cm), ie a diminutive woman and a large framed man, is 35.5cm (Adult Data 1998).
We cannot always design to fit everyone, so adjustable equipment and workplaces aid us in ensuring a better fit.
Further information is available in the book , referenced below.