FFT and number of samples relations
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.