It has two. One pre-cat, and one post-cat. Depending on the code, specifically, it will tell you whether it is the primary or secondary O2 sensor. With the O2 sensor bad, you're looking at running into shitty gas mileage eventually. You can throw some fuel injection system cleaner in your tank to possibly clean it a tad bit. It will likely come back though. You can get an O2 sensor from autozone or wherever you go for like $50 new.
up stream is the primary or the one that you can see from under the hood on your exhaust manifold
down stream is the secondary sensor that is on the drivers side of the catalytic converter... pray it isn't this one if your car has seen a few years of NY snow and salt bad things may occur.... cause my wisconsin one is STUCK!
Mine's stuck and I might need a new cat... for direct fit... plan on saving $500.00 + for the cat alone!