Does the number of samples matter for FFT, and how to get a specific frequency visible?
The "information content" is related to the number of samples.  The length of the FFT merely interpolates the spectral frequency curve represented by the number of samples.  If an FFT result bin doesn't end up exactly centered on some desired frequency, you can interpolate it from the nearest bins, using a high-quality (Sinc kernel, et.al.) interpolator, since FFT bins have a greater-than-zero bandwidth (the shape of the transform of the default rectangular or other window).  Using a longer zero-padded FFT gives you more interpolation points, and thus a denser looking point plot, but no better spectral separation resolution than provided by the number of actual samples.