Walls and infill % depends on your material and requirements.
Mine in the photo is printed in PLA, 0.4mm nozzle, 4 top, 4 bottom, 3 walls, 15% triangular infill.
Corner brace designed with the advantages and disadvantages of 3D printing in mind. (Most other brackets are designed to mimic what a CNC machined bracket would be, or wood joined together!)