How many O2 sensors are on a 99 Boxster?
 

How many O2 sensors are on a 99 Boxster and where are they located? If you cant tell me were their located then i just need to know how many i need to order?

There are 4 O2 sensors total. One before each cat and one after.

But you don't have to replace all 4
 

let the codes the ECU is sending be your guide.

And, if it is saying there are problems with all 4, then there may be a problem like the AOS that is causing the sensors to be contaminated.

And, while I'm at it, you can use the Bosch part numbers rather than ordering Porsche labeled parts...since Bosch makes em for Porsche.

Part numbers here