USB and firewire mixers/interfaces

Sorry if this is stupid. Just been pondering how it all works.

In relation to USB or firewire mixers/interfaces, what is the channel count available? 
IE: I know Adat is limited to 8 channels per pipe. I was wondering is there some guideline for USB and firewire.

In other words, if you have a USB or firewire equipped mixer, how many actual channels from Sonar can be accessed at once on the mixer?