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1. My 2000 S has four, one prior to each pair of cats and one after each pair. So, two per side.
2. They are sold as single units.
3. Normally the fault code downloaded from the DME is very specific about which O2 sensor is out of range.
4. Some people say to repalce al four at once, others just do one at a time for reasons of cost. I would start with one, and if it doesn't fix the problem, at least you didn't buy three sensors you didn't need.
5. The Boxster can perform fine with a bad sensor, but you could damage the catalyst on that side if the engine runs too rich on that bank. Catalysts are not cheap to replace! And who wants to stare at a check engine light all day???? Not me....
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