1991 Chevy Caprice oxygen sensor

i have a 305 motor, and it has a custom exaust on it. i purchased it with it. and i wanted to change the oxygen sensor, when i went and got it from the auto part store, it was two, but i only can find one on the car. can you help me ? how many is the car suppose to have? its a 1991 chevy caprice 305 motor, and if it suppose to have two will drilling a hole into the pipes and installing it , will it work like it suppose to?
thanks richard