I'm gonna replace my sensors and I am trying to figure out how many I need. Is it one per bank or 2 per bank, one up stream and one down stream?

 

You have two "Expensive ones" near the manifold and two after the Cat, I strongly suggest you get a code reader to determine which ones they are as they are expensive, you will also need to drop the exhaust to change them. hope this helps, I've had all my changed and they front ones were $250 each and the backs cost $100 each.