Please find the answers here:
You can capture 8 channels digital simultaneously at 19khz 24bit. But if you want to record the sounds, please watch the bus bandwidth to the storage. When you record 8 channels of 192khz 24bit, you need to have the bandwidth of 192kHz x 32bit x 8channels = 6.144 MBytes/sec to the storage. In many cases, the bandwidth of SD cards is not stable to keep the performance due to the mechanism of the file system. If you want to capture stable sounds, you need to measure the actual performance of the storage that you are using. Please check the throughput from microphones to your storage beforehand.
It depends on the system. If you will use the same characteristic of digital microphones, I believe it can keep the phase coherence.
Please see the connection of digital microphones and Spresense below:
As you see, Spresense provides the same clock to digital microphones. Please see the capture timing of microphones below:
The data at the same timing from microphones are serialized at the same time. It means if the outputs from the digital microphones are generated at the same time based on the clock provided from Spresense, it will keep the phase between the microphones.
I hope this helps!