How do I get the dsPIC33EP32MC204 to sample 8 ADC channels at 500 ksps?
The dsPIC33EP32MC204 is capable of simultaneously sampling eight inputs, but NOT in 12-bit mode. With 12-bit resolution, each channel's conversion must be started by program control. While it is possible to sample 500 ksps under program control, the ability to achieve that result depends on the application code.
"Section 23.1.2 12-BIT ADC CONFIGURATION" of the device datasheet and "Section 16 Analog-to-Digital Converter" of the 16-bit family reference manual show examples of optimizing the ADC throughput.