The datasheet describes AD7606 as sampling at throughput rates up to 200
kSPS for all channels. It is not clear if this means 200 ksps for each of the 8
channels or if you
need to divide the 200 ksps by 8.
The core ADC inside the AD7606 is > 1 MSPS so this is shared across all 8
channels. The cycle time to have all 8 channels converted is 5 us (depending on
ser/par interface). So it is 200 ksps for each of the 8 channels(You do not
need to divide the 200 ksps by 8).