Hi Anish,
The ADS8528 has four 'convert start' inputs, you can tie them all together and send your external trigger to those inputs to start the simultaneous conversion of all your input signals, there are eight independent ADC's inside the chip so everything happens in parallel on that side.
For the conversion data output, if you use the parallel interface, you simply read the data out by strobing the RD input. The latency would be how long it takes you to process that read sequence. The same is true for the serial mode of operation.