as far as recording bandwidth tlw is corrent. it's doubtful anyone will ever need USB3 just because the bandwidth is too low for USB2.
doing the math (assuming recording and output at 24 bit / 44.1kHz sampling rate - (also assuming true 24bit recording, so zero noise floor from the hardware)
24bits X 44100samples/sec = 1.0584Mbits/sec for 1 channel input only
you need an overhead of 2 channels of data stream for output (stereo output), so the output channels need 2.1168Mbits/sec for output. unless you run more than 1 stereo pair of output simultaneously, this number won't change.
USB2 bandwidth = 480Mbits/sec
output = 2.1168Mbits/sec
total bandwidth - output = 477.88Mbit/sec
that's the number we are left with to calculate how many channels of input we can have.
477.88Mbit/sec divided by 1.0584Mbit/sec/channel = 451.5 simultaneous channels you can record.
this is theoretical, of course. but even if you consider half of that saying your system (and by system I mean the complete path in and out of the soundcard and computer) is only 50% efficient, you'd still get over 200 channels you could record simultaneously on USB2.
I think most audio hardware companies consider the "safe limit" which they will not go above to be 127 channels on USB2 and 72 on Firewire 400.
THIS is the main reason we've not seen many USB3 soundcards. yes, there are some available, but the bandwidth of USB3 is complete overkill for recording. they're only offering the USB3 connections because of consumerism ("bigger is always better!") and/or convenience.