Funny you should link that! I have gotten as far as getting a raspberry pi zero to be a USB audio gadget, but I haven't played with the Pico yet. The raspi zero has one i2s input and one i2s output.
> Pico itself can interface 29 microphones with no sweat.
I... had no idea. I thought that since it didn't have an i2s peripheral I was going to have to either find a micro that did, or do something bitbangy using SPI and perhaps an external buffer. I see that it might be possible to get a few I2S inputs using the PIOs. Thanks for this, will certainly give it a shot.
(though I don't see how you're getting 29 microphones :P "prove it" ;)
Main limitation is USB 1.1 IO, so ~1MB/s, unless you are fine with recording to SDcard then probably around 10MB/s. Pico itself can interface 29 microphones with no sweat (30 GPIOs, 2.0 GB/s internal bandwidth).