The cartographer or map maker selects a map scale based on the size of the area they are mapping and the size of the sheet the resulting map gets printed on. It's common to find map scales ranging from 1:1 all the way to 1:10,000,000.
The tools we sell, get printed in large batches, and we only do that for the popular map scales.
I am slowly expanding the number of scales that I stock. If you have a common scale for maps in your area, that you think I should stock, let me know, and I'll add it to the list under consideration.
In the past several decades, mapping projects done for an entire country are usually done by a government agency. Typically these agencies will select one or more scales to use for their map series. Using a consistent set of scales allows the maps sheets to be used adjacent to one another.
Cartographers making a single map of a specific area, like a park, often choose a scale to fit the area onto a reasonable or standard sized sheet of paper. The National Geographic Trails Illustrated map series is a good example. They use more than 60 different map scales in their series of several hundred maps.
As maps and map production move into the digital world it becomes very easy to adjust the output scale to satisfy your particular needs. Whenever it is reasonable to do so, you should stick with the more popular scales for printed maps. This will allow users of the map to use the map along with other printed maps of the area, as well as allowing them to use the common coordinate and distance measuring tools.