I am thinking of changing the o2 sensors, something I have been meaning to do for awhile. I just don't know how many sensors there are? I bought two (unheated) which is what autozone told me i needed only two, however, I couldn't find anything in the hayne's manual, and i have seen some diagrams online that show four (heated) sensors, two by the manifolds, two further down.

Any help/clarification would be appreciated.

91 vintage VIN L will only have one O2 sensor in the rear manifold just before the spring loaded flex joint at the pipe. Don't know if it's pre-heated on that particular model, although I believe 91 predates the heated sensors.

91 vintage VIN L will only have one O2 sensor in the rear manifold just before the spring loaded flex joint at the pipe. Don't know if it's pre-heated on that particular model, although I believe 91 predates the heated sensors.

Yes, you've already found the only O2 sensor on your car. It should be just a plain old single-wire sensor. Heated O2 sensors have 3 or 4 wires and were first put on the Regals in 1993. Downstream sensors were added in 1994 IIRC.