As a feel for what resolution is theoretically possible, consider a 0.1s sampling at 48kSPS of a 1kHz tone.
Thats about 5000 samples.
That will have 100 cycles of the fundamental, ie 200 zero-crossings. If the sample depth is 12 bits, you
might be able to linearly interpolate a zero crossing to about 1/200th of a sample period, ie 100ns.
Averaging of those crossings refines the estimate by the square root of the number of crossings to about
6ns, which as a fraction of 0.1s is 0.06ppm, ie a frequency resolution of 60 micro hertz.
This is, of course, completely unrealistic unless the original tone is ultra stable and has ultra low distortion.
However just by rounding zero-crossings to the nearest sample you get 1 part in 5000, ie 0.2Hz
resolution. So for 1s sampling time you can get 0.02Hz resolution or better, without super high constraints
on the cleanliness of the signal.