Why do I get a high peak in my FFT plot of a 50 Hz sine wave?
It all looks normal to me.   By Parseval's theorem, the sum-of-squares of the input function's samples is equal to the sum-of-squares of the transformed values.   A comment indicates 'it gives the power', but the sum-square of those values
is NOT the power, it is the ENERGY.   Power, in the transform, is
the square peak height times the bandwidth (i.e. an area under a peak).
By taking multiple cycles, the bandwidth is diminished and the peak
height goes up.