Hi Feras,
as you know #FFT is the maximum number of the Nsc, so when you set the #FFT to 64 and Nsc to 16. that is mean just 16 of the #FFT will be active or use to transmit the data and the other will be zero, but the spectrum will be consider all #FFT, that is why spectrum BW= Fs*#FFT.
you can try that, for example set the #FFT to 12 and the Nsc 4. then after you run the simulation go to report>>go to the OFDM modulation component>>graphs>>OFDM FFT. you can see the spectrum and all the #FFT point including the Nsc