V6 or V8 engine will have one O2 sensor on each exhaust pipe just after exhaust manifold, these are called "upstream" O2 sensors so there will be two, one on each "bank", these would be near Bellhousing.
These help computer adjust the fuel trim, if there is too much oxygen then burn is Lean, too little oxygen then burn was Rich.
In 1995 a 3rd or 4th O2 sensor was added.
These test if the Catalytic Converter(s) are working, they are called "downstream" O2 sensors, they would be located after the converters so closer to the trans/driveline area.
If you have a single exhaust/tailpipe then there would usually just be one downstream O2 sensor.
If you have dual exhaust then there would be two, one on each pipe.
On some newer vehicles I have seen two downstream O2 sensors on a single exhaust, one after first Catalytic Converter and then a 2nd O2 after 2nd Catalytic Converter, so each Cat was being tested.