There's 2 on each pipe but they act as 1. You can install High Flow cats like the JBA set pictured below that would only have 1 on each pipe. Some people install catless mid pipes and eliminate them all together, typically with LT headers.
If you're popping a code and you've confirmed you've got a bad cat, you will have to replace the entire pipe with the 2 on them. Not recommended you cut one out and just replace one and leave the other on the same pipe.
Stock cats:
High flow cats:
