Why did you measure 20 cycles of the pendulum motion to determine the period, rather than just one cycle?

This is basically to avoid inaccurate period of oscillation.

Explanation:

Period of oscillation (T) is calculated by dividing the time of oscillation by the number of cycles.

Example; if 1 cycle is used to determine the period of the oscillation, the timing will be inaccurate and there would be error in estimating the period of the oscillation.

For a better and accurate result, a larger number of cycles is taken, the timing will be accurate and the period of the oscillation would be estimated by dividing the oscillation time by the number of cycles. This minimizes error and gives a better estimate of the period of oscillation.

Example; if 20 cycles of the pendulum motion takes 12 seconds, then the period of the oscillation is 12/20 = 0.6 seconds.

Thus, 20 cycles is used to obtain accurate period of oscillation.