How many O2 sensors does my 2011 Hyundai Veracruz have?

You will have a sensor before and a sensor after the catalytic converter. Most of the time, a P0420 code sets as a result of an inefficient catalyst -- meaning the catalytic converter is not working properly. The engine's computer will monitor the composition of exhaust gases before the catalytic converter, and again after it. If it notes no change or little change, the light will come on. Sensor replacement is most likely not going to help you. It might be worth checking on your vehicle warranty; catalytic converters typically have a federal emissions warranty that may be 7 years / 70,000 miles.