To convert one sample to ADC = ACQUISITION TIME + A/D CONVERSION TAD CYCLES
As per datasheet ACQUISITION TIME = 19.72us
A/D CONVERSION TAD CYCLES = 12 * 1.6us = 19.2us
Also consider, Holding capacitor is disconnected time 100ns
This is the minimum specified values to be considered, but it is always better to allow some higher time for both Acquisition time and conversion time
Now you have to take in to account, what kind of action that can be taken for the result and time before take in to account next sample, other wise result registers will overwrite
Also note that the ADC operation is Independen operation