Effect of the horizontal magnifier control on the sweep speed, sweep time and number of cycles observed?
A horizontal magnifier on an oscilloscope is designed to decrease the sweep speed to allow better viewing of high speed signals   For example, assume the sweep speed is 1 usec/division.  Applying the 10X horizontal magnifier will decrease the sweep speed to 0.1 usec/division (It  multiplies the horizontal gain to achieve this). Thus you will see less cycles of your signal but at a higher sweep speed so you can see more details.