Question

iMac and Garageband recording.

Answer 1

You need 16 pre amp channels and 16 analog to digital channels to do that

There are products that bundle the two together.

Here is a 16 in/16 out analog/digital interface:

https://www.sweetwater.com/store/detail/AuroraN16USB--lynx-aurora-n-16-usb

I think you'll have more options if you look for two 8-channel pre-amps rather than one 16-channel pre-amp:

https://www.sweetwater.com/c662--8-channel_Preamps

Hope this helps,

Steven