The analog input will use separate ADC modules, just as the analog output uses separate DACs. DSPi itself is purely digital (OK, excepting the PWM based sub out). These modules are just a few dollars on AliExpress for ~96dB SINAD

Yes, but this project doesn't do anything analog to begin with. It could just have several S/PDIF and I2S inputs, and convert that to USB. You probably don't want any processing then, and just pass the digital inputs straight to USB. The limit of how many channels you could simultaneously process would then be the USB bandwidth.

Even worse, the ENOB is closer to 9 bits in testing. It’s got horrible DNL/INL. Totally worthless for any audio unless you’re trying to do chiptunes or something.