FFT and number of samples relations I am new in signal processing. I generated a signal with $f_{in}=10 \mathrm{kHz}$ and also take 64 sample from this signal after doing some process in an ADC block. I want to convert the result to the main signal and compare them whit each other. So if what is the difference between taking an FFT of the 64 points or other numbers like 1024 in the same measured time and what should be changed?

Thank you
It is not clear from your question why you are even taking an FFT; knowing that may also lead to additional insights- but the main point to understand is frequency resolution in the FFT; assuming you are not doing any windowing (shaping the envelope of the time domain signal before taking the FFT), the frequency resolution is 1/T where T is the length of your FFT block in seconds. From this you will see that the frequency resolution is one FFT bin, and is equal to $f_s/N$ where $f_s$ is the sampling rate used and N is the number of samples in the FFT.

For additional info on this see What happens when N increases in N-point DFT and the additional referenced posts there.