A big Q

1990 XJ Renix 4.0... My idle is going between 450-750. Up and down. After searching posts I am going to ohm my 02 sensor. My question is do I have two? The manifold and one past the cat or is there more. And if I find one bad do I change them all at once or only the bad one????

2 oxygen sensors started in 1996 with the introduction of the OBD-II computer system.......