A single box plot can be drawn for one batch of data with no distinct groups.
Alternatively, multiple box plots can be drawn together to compare multiple data sets or to compare groups in a single data set.
For a single box plot, the width of the box is arbitrary.
For multiple box plots, the width of the box plot can be set proportional to the number of points in the given group or sample (some software implementations of the box plot simply set all the boxes to the same width).