Assuming you have the V-6 engine, it has two downstream (after the cats) O2 sensors and two upstream (before the cats) Air/Fuel Ratio Sensors. So the answer to your question is two O2 sensors.
The 2007 Nissan Xterra has a V6 engine.
The 2007 Nissan Xterra has 24 valves.
The 2007 Nissan Xterra has a 6-speed manual.
The 2007 Nissan Xterra is a gas-powered vehicle.
The 2007 Nissan Xterra is 14 ft. 10.7 in. (178.7 in.) long.
The 2007 Nissan Xterra has a 30.5 degrees angle of approach.
The curb weight of the 2007 Nissan Xterra is 4150 lbs..
The 2007 Nissan Xterra has double overhead cam (DOHC).
The 2007 Nissan Xterra's turning circle is 37.7 ft..
The 2007 Nissan Xterra runs on regular unleaded.
The maximum payload of the 2007 Nissan Xterra is 1138 lbs..
The 2007 Nissan Xterra has a 27.3 degrees angle of departure.