How many 02 sensors are on a 1999 Chevy blazer?

A 1999 Chevy Blazer typically has one O2 sensor per catalytic converter. Since it has two catalytic converters (one for each bank of cylinders), it usually has two O2 sensors. However, some models or configurations *might* have more (e.g., if it has a pre-cat O2 sensor as well). To be absolutely certain, you should check your vehicle's repair manual or the parts diagram for your specific Blazer's engine.