The E90/92/93 3-Series model use a total of 4 O2 sensors Two before the catalyst and two after the catalyst. Select O2 sensor location from the menu. Made by BOSCH. OEM. Sold each.
Part numbers below are for reference.
Part # 11 78 7 545 074 Post Cat in Front Manifold. Cyl 1-3 (560mm length)
Part # 11 78 7 545 075 Post Cat in Rear Manifold. Cyl 4-6 (490mm length)