I called my supplier and i told him i want a front O2 sensor for a 96 Bonneville SE, and he tells me there 3 different types of sensors, one on the exhaust manifold & 2 before the catalic converter ???
its the one on the exhaust manifold no?

You want the exhaust manifold before the cat. Do not use Bosh. Check at Auto Parts Fast at RockAuto the prices there are very good.

great thanks

How many total O2 sensors are there? And what are their functions?

Some have one, and some have two. The front one tells the PCM if your rich or lean and it then makes changes. The rear one tells the PCM if the cat is working or not.