Spotting scopes are often referred to by numbers separated by an "x". For example: 15–45x60. The first number(s) indicate the power or magnification of the spotting scope. With a 15–45x60 variable power spotting scope, the object being viewed appears to be 15–45 times closer than you would see it with the unaided eye.
The majority of the spotting scopes have variable magnification power due to practicality and application. Fixed magnification spotting scopes tend to be more compact and cross over into monoculars.
If you consider magnification of optical devices as a spectrum, spotting scope magnification would fall to the right of binoculars and left of telescopes. The most common magnification range for spotting scopes is between 25x and 60x. You can still find spotting scopes with magnification ranging up to 100x or even beyond, but typically it comes at a price both financially and in terms of features/ functionality.
Spotting scopes like all optical devices are affected by many factors that often have nothing to do with actual optical design and manufacturing itself. Atmospheric conditions, such as heat coming off the ground, dust suspended in the air, humidity, wind currents could seriously affect image quality especially at higher magnification settings.
As far as the optical design is concerned, the higher the magnification the lower is the amount of light that gets gathered on the focal plane which in tern affects image quality. For most scopes in 20x-60x range this is not that significant for general observation, but it does become a factor when magnification values climb above that number.