The magnification that a telescope and eyepiece combination provides is a representation of their power. In most cases, less magnification is better for the simple fact that more light can be observed. The main problem you run into at high magnifications is turbulent layers in the atmosphere. These turbulent layers are more pronounced at high magnifications and cause light loss and blurring.
The magnification of a telescope of focal length ƒ_scope[mm], equipped with an eyepiece of focal length ƒ_ep[mm] will then be:
Magnification = ( ƒ_scope ) ÷ ( ƒ_ep )
My telescope has a focal length of ƒ = 650 mm and when using a 17 mm eyepiece I will have achieve a magnification of 38x.
Magnification = ( 650 mm ) ÷ ( 17 mm ) ≈ 38x