The General syntax for adding subplots is : axes_obj = fig.add_subplot(nrows, ncols, plot_number) .
The total number of subplots that can be added to the figure will be nrows multiplied by ncols.
Following this background,
The figure can only accommodate a total of 4 subplots.The code adds 4 subplots to the figure.
This creates 8 subplots but adds only 4 subplots that was defined in the code to the figure. The 8 subplots are created such that it has 4 rows and 2 columns but only the first 2 rows are fully occupied by the subplots added.
Therefore, in the block of code can still accomodate 4 more subplots as in: