O2 sensors question

2000 Blazer 4.3 73K. I am thinking about changing out my O2 sensors. My buddy at NAPA can get me a good deal on Bosch sensors. My question is which ones do I swap out ? It looks like there are more than 2 on here....

Thanks !

You should have 4.

I would change them all but you really only have to change the 3 before the CAT Convertor.

Thanks !