Typically I use no more than 22 or so. Even when doing full band live tracking. So I think 24 would be sufficient. 32 is nice just in case... ie...
I did this huge non-metal band with keyboards, acoustic guitars, timbales, congas, full drum kit with additional percussion, etc. Nearly maxed me out and I had to some rigging to get it all working.
So here is my max setup.
MOTU 828mk3
2 Mic/Guitar DI ins
8 analog line ins
2 SPDIF ins using my DBX Quantum as converters
8 ADAT with Digimax FS
8 ADAT with BLA modded ADA8000
So that is 30 typically. But the cool thing with MOTU is that you can slave additional units. So I was able to slave a really old 828.
8 ins (2 with pres) MOTU 828
in theory
2 SPDIF
8 ADAT
Now I think we may be taxing the firewire bus at this point. I definitely had to raise my buffers to even get this to run smoothly.
Even this band, we didn't "need" that many inputs, but they wanted to play live and be ready to switch instruments at any time. Definitely more of a "Commercial" studio expectation. I was surprised I was even able to handle it as a project studio. I was really surprised to have enough cables and mics (obviously not primo choices). We topped out at 36 simultaneous channels.
But hey, it ended up working. But definitely not typical, especially for a project studio. In my 7 years of recording, this was the first time I have very had to do that. Won't be the last unfortunately since the same band is coming back....