How many Oxygen Sensor or sensors does this vehicle have?

That car has 2 sensors on it. The upper sensor (part # 55569903) is visible in the top of the exhaust converter- manifold and the lower sensor (part # 12612430) is in a bung welded to the exhaust pipe that bolts to the bottom of the converter. Both are torqued to 31 ft lb if replaced. Are you having an issue where you think one might be faulty? If so what are the actual codes?

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

No Codes just have poor mileage and haven't replaced them with milage @ 130000. Do the oxygen sensors need calibrations?

No codes should mean that the sensors are okay. I would put a scan tool on it and look at what is actually going on before just replacing parts. It could be something other than sensor problems. I would also look at testing the battery as well.