If using the PJRC audio adaptor (2i2o), a maximum of 3½ boards can be used, but it gets increasingly tricky to do after the first two.
The "main" Teensy I²S interface (the various Audio???I2S objects) has a maximum of 5 data pins (7, 32, 9, 6, 8), and there must be at least one input (pin 8) and one output (pin 7) - the other 3 can be allocated to either role. The secondary interface (Audio???I2S2 objects) has just one input and one output (pins 5 and 2).
The other issue is the I²C control interface. The SGTL5000 can only have two addresses, so to use more than 2 audio adaptors requires use of another Wire port, which is not catered for by the AudioControlSGTL5000 objects so you're into editing the library.
The Rev D audio adaptor does have pads to
change the I²C address and
patch the data signals:
View attachment 36083
Either way, I²S is not a good choice for high I/O counts, you really need TDM. Even then, there's no easy way to get to 16 channels (I'm assuming you mean 16i16o here) at the moment.
The "get you going" option is to get a couple of the cheap CS42448 boards from AliExpress, which will give you 12i16o. It's not completely trivial to make this work, as they're not ready to go out of the box - you need to do some configuration to select their I²C addresses and add pull-ups. Again there's a limit of 3½ boards, but it's easier because they can have 4 different I²C addresses, so the control aspect is simpler; you could make a 24i24o system. A couple of relevant forum threads can be found
here and
here. I'd expect CS42448 boards to remain available for a little while, as last orders were June 2024 and last deliveries will be September 2025; after that stocks will run down, of course.
In the longer term,
this project looks as if it's going to be very interesting. Changing your software over to using that hardware should simply be a matter of using a different control object - all the TDM stuff will be exactly the same as for a CS42448-based system.
The
Design Tool info pane shows which pins are needed for the various clocks and data expected by each of the library objects you put in your design. That and the
audio adaptor product page give a wealth of information, and you need to read and understand them pretty thoroughly to get a successful outcome.