Typically, there are two; upstream and downstream (catalytic converter in between both).
Just going by your year and model, the dealership parts diagram shows two. However, the parts list shows a left and right upstream and a left and right downstream, which implies there are 4 sensors. To complicate matters, there are 4 different upstream and 4 different downstream part numbers.
Just from experience, if/when buying Toyota parts it best to go by VIN and not year and model. Oh yeah, have the date of manufacture handy. This goes for Lexus as well.
Ya, upsteam front downsteam rear, upstream right downsteam left, four different O2 sensors. Use to be just one, they are extremely slow in responding, takes a good microcontroller to iron things out. Big job is to keep the stoichiometric ratio constant. Too rich, hydrocarbons in the air, too lean ozone. So the EPA was not happy with one, wanted two, one directly in the exhaust manifold, the second on behind the catalytic converter.
Assuming your vehicle has dual exhaust?
Any misfire, spark plug, little water in the gas will trigger an O2 and if taken to shop, depending on the honestly may say your O2 sensors or maybe in your case, all four should be replaced. They are made much better, back in the 80's when they first came out, about 25 K miles was the maximum life. They control the dwell of the fuel injectors. The rear one is suppose to track the front one, but if you get a real O2 code, really don't know which one is bad. Can be tested with a high impedance VOM for about 6-8 pulses per minute.
Nick you're right, Toyota 4 cyl, with four sensors talk about REDUBMDANT! wide band and narrow band and how about almost $95 to $488 bucks to boot, no wonder there are so many good looking Toyota's in the junk yards.
They charge so much for O2 sensors because of the direct fit, that's the plastic connector on the end of the wire. No industry standards for that!
It's kind of like saying the engineer had a D A attack ((dumb ass attack)) when they designed a lot of the systems that require a huge amount of work to fix, like heater core replacements that require entire dash removal. Well a hvac module install saved five minutes at the factory but takes 12 hours to replace just the heater core in the aftermarket but then again warranty never sees one.