Each objective is labeled with its magnification, for instance 5x or 100x.
However, the magnification of the objective alone does not determine the overall magnification of the microscope. This results from the objective magnification multiplied by the eyepiece magnification (for tube lens 1x).
40x objective x 10x eyepieces = 400x overall magnification
It should be noted, however, that the higher the objective magnification, the lower the visible object field.