Each map on this online shop has its scale (e.g. 1/20 000) which is the true scale at the equator, with the actual print size (0,5 m) for the tile.
The real scale for your map is actually different. It is dependent on the latitude of the area that is projected in Web Mercator.
For example: your map is Belgium | 08. The given scale for this level is 1/300 000 at the equator. Belgium is located between 49.5 and 51.5 degrees North. The actual scale for this country on level 08 is approximately between 1/195 000 and 1/185 000.
Calculate the real scale for your specific location by entering the latitude degrees: