how many O2 sensors?

How many o2 sensors are on a 2002 1500 van V6? first one place says 3, other says 2 another 4. Like to save money and try to change it myself but I have to find the location of sensor. Code is PO138 O2 sensor bank 1 sensor 2. I kinda looked around a seen 1 in manifold drivers side and 1 after the Y in exhaust pipe which shares exhaust from both banks and its before converter and 1 after converter. do I need to change the one after the converter? is that bank 1 sensor 2? anybody know a website that has diagram location of these sensors? thanx :icon16: