thank you for your corrections!
the readme.pdf says google-translated from German
'With the PIO's you can connect up to 8 additional UART interfaces (
alternatively as set up a transmitter or receiver)
on freely selectable pins of the PI PICO, in addition to the two existing interfaces.'
So up to 4 additional full duplex UARTs are possible with the source as it is now (4x RX channels + 4x Tx channels).
From the PIO setup code it seems that one PIO can handle 4 channels,
so 8 channels would use
two PIO units:
PIOx_BASE = 0x50200000 + ((smNumber >> 2) << 20)
PIOx_RXFx = PIOx_BASE + 0x20 + ((
smNumber & 3) * 4)
However,
https://www.raspberrypi.com/products/rp2040/specifications/ states the RP2040 PIO specs:
'
8 × Programmable I/O (PIO) state machines for custom peripheral support'
So I still get the impression that 16 additional full duplex channels would be possible using all 8 PIO state machines.
---
It seems that each channel can have its own baudrate, for it is a separate initialisation parameter for each channel.