Magnification is the easiest number to calculate on a telescope and the most misleading to shop by. It is entirely set by which eyepiece you push into the focuser, which means any telescope can claim any magnification, and the ones that advertise the biggest numbers are usually the ones least able to use them.
The calculation, and an example
Divide the telescope's focal length by the eyepiece's. A 300mm instrument with a 20mm eyepiece gives 15x. Swap to a 10mm eyepiece and you get 30x. Add a 2x barlow to the 10mm and you get 60x. Nothing about the telescope changed and the light it gathers did not change either: the same photons are spread across a larger, dimmer image.
The useful limit, which is set by aperture and air
A telescope can only resolve detail down to a limit set by its aperture, and magnifying past that point enlarges blur. The working rule is about two times the aperture in millimetres as the maximum on a steady night: 150x on a 76mm instrument, 260x on a 130mm. Atmospheric turbulence usually intervenes first, and on many nights 150x is the practical ceiling regardless of the instrument.
What magnification to actually use
Most observing happens between 30x and 150x. Low power finds things and frames clusters; medium power is where most planetary detail appears; high power is for splitting close double stars and for the moon on the steadiest nights. Experienced observers reach for high power last, not first.
Questions people ask about telescope
How do I calculate telescope magnification?
Telescope focal length divided by eyepiece focal length. A 1200mm telescope with a 25mm eyepiece gives 48x.
What is the maximum useful magnification?
Roughly two times the aperture in millimetres on a good night, and often less because of atmospheric turbulence. Beyond it the image gets larger and softer.
Why do cheap telescopes advertise 525x?
Because magnification is a property of the eyepiece and any instrument can reach any figure on paper. A 60mm telescope at 525x shows a large dim blur, which is why the number is a warning sign rather than a feature.