Just one is needed. They are dual channel op amps.
I swapped out the op amp with the Brown Burr and gave it a listen, not critical but didn't hear anything that caused concern. For the most part, swapping this out is like an adult version of Operation. If your intent is to let people "roll" their own, perhaps a socket that is a little easier to use. I've done this before with a Sutherland phono preamp and the sockets in that until had more user friendly opamp sockets.
Waiting on a few other changes to my system to settle down before I give a critical review.