Miamicarfan

Is it more than one?

Also, what are their prices if bought at the dealer?

poltergeist

One air/fuel ratio sensor (similar to an O2 sensor) before the catalyst, and one O2 sensor after the catalyst

PACman

There's the primary O2 sensor (located on the header)...techinically called the air/fuel ratio sensor
019 36531-RBB-003 SENSOR, AIR FUEL RATIO 1 2004 TSX $213.33

Then there's another O2 sensor (on the catalytic converter) following the primary O2 sensor
020 36532-RAD-L11 SENSOR, OXYGEN 1 2004 TSX $203.33