Every enclosed area on the sheet carries a number, and the key along the foot says which of six colours it takes. Six, not twenty: these are meant to be finished with the pencils in an ordinary tin.
The numbers are placed by finding the areas rather than by labelling the shapes that were drawn. Where two outlines cross they make a new area between them, and it is that area that gets a number, so a child never has two numbers for one patch.
That is the same method the build uses to check a coloring page holds ink: flood everything the lines do not cover, and count what is enclosed. Here the result is used to draw the page instead of to test it.
Areas too small to hold a legible numeral are left blank on purpose, rather than printed with a digit sitting on its own outline.