What frequency should an oscilloscope be rated at to measure USB signals
12 Mbps is equivalent to a square wave of 6 MHz when the bits are toggling at maximum rate. 

However, if you are wanting to measure rise times and fall times to any degree of accuracy you are going to have to dig deeper and put more info in your question.

If you are not too fussed about rise times and fall times and just want to capture streams of data for logical analysis then a 100 MHz sampling scope is good enough and you might get away with decent results at 50 MHz sampling.