how many converters need to be replaced and how much

There are three, two front and one rear. It will likely cost you more than $2,000, do you really want to do this? If so, your car better be in remarkably great condition, like a museum piece. I like older cars, but, it's got to be pretty special to do this reconditioning. Good luck. Find a good independent mechanic.

http://www.autozone.com/repairguides/Subaru-Car-1998- 05/Component-Locations/Heated-Oxygen-Sensor-H02S/_/P- 0996b43f8037aa9c This is showing TWO and that piece of pipe with two CC's, (with gaskets 217) and another is 229. Consider a pretty decent mark up at chain shops (so you need to shop around on price) and add in two new gaskets (5-10 bucks tops) and MAYBE one hour labor at the max.. I would think 400-500 TOPS!!! I would consider buying a decent ratchet and metric sockets and do it yourself. Google shopping: 2005 subaru outback catalytic converter