55 amps is 55 amps. One or more rails - it shouldn't matter.
I can't confirm that PSU can actually provice 55 amps of +12V. There isn't enough data on the data plate to be sure. You can't just add up each +12V rail - you have to known the max +12v the PSU can actually produce.
For comparison here is the Corsair 750TX data plate (and it happens to be less expensive):