how many o2 sensor are there?

[GuSpeed]

I am planning on using Seafoam and changine the spark plugs. Now how many sensor do I need to change? Not exactly sure what sensor are there and what needs to be change.

[L0sT]

there SHOULD be only one o2 sensor, which should be sticking out of your exhaust manifold

[oakleys_s13]

so if you are just doing a tune up then you should be able to get out of just one o2 sensor