The range is 1-10. If you keep clicking the up/down arrows it will stop when it reaches the end of the range, so that's another easy way to verify this for any of the settings in the UI. It won't allow you to input numbers that exceed the range.
If you hover your mouse over the setting it displays an additional explanation that explains more about it. It says the soothing level is the "intensity of the smoothing", so larger numbers would smooth the layers more (less abrupt transitions), and lower values would allow faster changes (more abrupt).