everthing i need to knwo about them what they do and how meny are in a 90 iroc z

They regulate your fuel and air mixture, which should be 14:7.1. I just changed mine, the front one by the exhaust manifold. The check engine light will come on if it's really bad. (You can check the code with a paper clip to flash the light, if it is on.) But it can be going bad long before the light comes on. I'm going to add mine to my tuneup schedule now.
My car has better response, more power and is getting better gas mileage with a new one. Go Delco for the replacement. Pricier, but what the car needs and should get, IMO.
I read a lot before changing it. Google replacing oxygen sensor.

It senses how much unburned oxygen there is in the exhaust. It can tell by how much oxygen is left whether or not you're running rich or lean. For all of your fuel to burn, it must have oxygen in the correct (stoichiometric) amounts. As mentioned, 14.7:1 is what the aim is.

The computer reads how much oxygen is unburnt in the exhaust via the sensor and adjusts how much fuel it's putting into the car accordingly.

If your oxygen sensor is bad it will give incorrect readings and the comptuer will dump incorrect amounts of fuel into the motor. This can result in minor issues usually, since there's so many other sensors the computer is using to meter air and fuel at the same time, but it wont run its best, and if it gets really bad it can cause all sorts of problems. The car will just run like it's not tuned properly.

Now I am going to butt in and say Walker Oxygen Sensors are better than the stock AC/Bosch.