First post [newb]

As noted from the title I own a 91 Honda civic wagon 4wd (think wagon attack). Anyways I have owned the car for a year now. Has 90k (replaced crate engine)
The red light (passenger light) has been flashing 1 flash. I have looked online and note its the oxygen sensor. I have located it on the engine, but my question is there only 1 sensor or 2? Upstream/downstream? I plan on just replacing it myself. Second question is regards to before the o2 sensor code blinked I had a code 7 for the TPS I even have located that but know its a little difficult to get off? Has anyone worked on these before, help? I have this guess that the TPS is associated to the o2 going bad? What do I need to look for with symptoms, advice? Thanks