How to interpret microcontroller ADC module on datasheet? Datasheet syntax question
Your misunderstanding lies in the fact that "ADCClock" is not the "sampling rate", it is the internal clock with which the ADC module on the microcontroller works:

"Converting a single analog value to a 10-bit digital data requires sample and hold clock cycles as defined in the Sample and Hold Time field of the Mode Register (MR.SHTIM) and 10 ADC Clock cycles." (section 29.6.1, p. 792)

In other words, to get a single sample from your ADC requires actually quite a few (a minimum of 13 if my cursory read of the data sheet is correct) ADC clock cycles, and not just one. Your sampling rate is simply a function of how (often) you trigger a conversion (section 29.6.5). For this you would probably use one of the timer modules, which will most likely give you more flexibility in the actual intervals so you can achieve your 8kHz target rate.