Being Curious How many sensors in an 05?

There are lots of sensors since the Prius has several electronic systems that require feedback for proper operation.

Regarding the exhaust system, there is an air/fuel ratio sensor and an oxygen sensor. Those provide input to the engine ECU so that the proper fuel/air mixture can be maintained and so that the health of the catalytic converter can be monitored.

If you have HID lights, there is a sensor mounted to the rear beam axle so that as weight is added to the rear of the car, the headlights will point downwards to prevent blinding oncoming traffic.

Since your car has needed multiple oxygen sensors, I think it would be reasonable to inquire into why this is the case? Note that for example, the presence of antifreeze might cause early oxygen sensor failure. Have you noticed any problem with your radiator coolant level dropping?

The following Wikipedia article may be of interest:
[ame]http://en.wikipedia.org/wiki/Oxygen_sensor[/ame]

"Heated sensor lifetime is typically 100,000 miles (160,000 km). ... For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion, similar to the reason a battery stops producing current. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the mileage worsens.
Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.
Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.
Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response.
An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system."

 

A couple of years ago there were problems with silicones getting into the fuel supply for a few supermarket filling stations.

A splutter, a judder and a hefty repair bill | UK news | The Guardian