ok maybe on the newer models but on mine theres only 2 sensors. both are mounted ontot the exhaust manifold (header). one 02 sensor is before the cat on the stock manifold and helps to regulate air/fuel ratios and stuff, the other is after the cat and read the exhaust fumes to let you know if you can pass emissions or not. basically the second one lets you know that your cats working ok. both of these are before the second cat and there are no more sensors after the second cat.
i think the pzev or something models alng with the 06 and up have more sensors.